What is familial adenomatous polyposis (FAP)?

FAP is a rare condition that can run in families. It sometimes used to be called Gardner syndrome.

FAP causes hundreds or thousands of small growths in the the large bowel. These are called polyps or adenomas. They usually start to appear when a person is in their teens. 

If the polyps are not treated, 1 or more of them will almost certainly develop into cancer. This usually happens by the age of 40. To prevent this, many people affected by FAP decide to have surgery to remove the large bowel.

FAP and other parts of the body

FAP can also affect other parts of the body. The effects depend on the type of gene variant. They may include:

  • harmless black dots on the back of the eye called CHRPE (congenital hypertrophic retinal pigment epithelium)
  • harmless lumps on the bones called bony osteomas
  • harmless lumps on the skin called sebaceous cysts
  • polyps in the stomach and the first part of the small bowel (duodenum) – these may develop into cancer
  • a type of benign (non-cancerous) tumour called a desmoid tumour
  • more rarely, cancer in other parts of the body.

FAP and the APC gene

Your body is made up of tiny building blocks called cells. Inside every cell is a set of genes. These genes are the instructions the cell needs to work properly. If a gene is changed, it may not give the correct instructions anymore. A change in a gene is called a gene variant or mutation.

People with FAP have a variant in a gene called the APC gene.

How FAP runs in families

Genes are passed from parent to child (inherited) when a sperm and egg join to start a pregnancy (conception).

We have 2 copies of each gene – 1 from our mother and the other from our father:

  • the sperm contains 1 copy of the father’s genes
  • the egg contains 1 copy of the mother’s genes.

If 1 parent has an APC gene variant, you will get either the copy containing it or the copy that does not. There is a 1 in 2 (50%) chance the gene variant is passed on.

How genes are inherited

Parent and child figures are used to show how 4 combinations of genes can be made from 1 sperm and 1 egg. 2 of the combinations contain a gene variant carried by 1 parent.
Image: The image shows a female and male figure. A pair of genes is shown beside each figure. The female has two green genes shown side by side. The male figure has a green gene and a red gene shown side by side. The red gene is the gene variant. Below the female, two circles are shown. These are two eggs. Each egg contains a copy of one of the female’s genes. Below the male, two ovals with tails are shown. These are two sperm. Each sperm contains a copy of one of the male’s genes. One sperm contains the green gene. The other sperm contains the red gene variant. Lines travel down from each egg and sperm to four child figures. The lines show which genes have passed from an egg and a sperm to each child. Four possible combinations of genes can be made using one of the eggs and one of the sperm. Two of the children have two green genes. They have a copy of a green gene from one of the eggs and a copy of the green gene from the sperm. Two children have a green gene and a red gene variant. They have a copy of a green gene from one of the eggs and a copy of the red gene mutation from the other sperm.

About 1 in 5 people with FAP (20%) are the first in their family to have the gene variant. This means your parents did not have it. Any children you have will still have a 1 in 2 (50%) chance of inheriting it from you.

Diagnosing FAP

FAP is usually diagnosed by genetic testing. A sample of blood is checked for an APC gene variant.

If you have a parent, brother or sister with FAP, you can have this test to check for the variant they carry. It is usually offered from about the age of 10 or 12. Your GP should refer you to a specialist genetic centre for this test.

You may have a relative who has hundreds of polyps, but genetic testing has not found an APC gene variant. In this case, a genetic test will not be possible for you. Instead, you will be offered bowel screening tests to look for signs that you may have FAP. 

People without FAP in their family may be diagnosed after tests or treatment for something else. For example, you may be offered genetic testing if a high number of bowel polyps are found during:

Understanding genetic test results

There are different APC gene variants that cause FAP. Many, but not all, of them carry a very high risk of bowel cancer. There is a milder version of FAP called attenuated FAP (aFAP). This can also be caused by variants in the APC gene. The effect of FAP on other parts of your body may depend on the exact gene variant you have.

Sometimes a person has hundreds of bowel polyps, but genetic testing does not find a gene variant. This may be caused by a gene variant that experts currently do not know about. The person is usually treated as though they have a diagnosis of FAP.

Your genetics specialist will explain:

  • what your genetic test shows
  • how this is likely to affect your body
  • how this may affect other people in your family.

We have more information about test results and genetic testing.

Screening and FAP

You may be offered screening tests if you have an APC gene variant or one of your first-degree relatives has FAP. First-degree relatives are your parents, brothers, sisters and children. These tests aim to:

  • find cancer at an early stage when it can be treated most effectively
  • find and treat changes that are likely to develop into cancer.

Your doctor will explain which tests you need, when and how often. It is important to keep having screening unless genetic tests show you definitely do not have the gene variant that affects other people in your family.

Bowel screening

You will be offered regular bowel screening with a test called a colonoscopy. This test checks the lining of the bowel from the inside. If the number of polyps in your bowel is increasing, you may need surgery.

Stomach and small bowel screening

You may be offered a test called an upper endoscopy to look for early signs of cancer in the stomach or small bowel. Your specialist can tell you how often you will have this test.

Screening for other cancers

Your risk of other types of cancer linked to FAP is much lower than your risk of bowel cancer. There are currently no effective screening tests for these other types of cancer.

Sometimes screening for other types of cancer is offered as part of a clinical trial. Your doctor can explain whether this is available. They can also explain the possible risks and benefits of taking part in a clinical trial.

Bowel surgery for FAP

Without surgery to remove the large bowel, most people affected by FAP will develop bowel cancer. Your doctor will talk to you about your risk and the best age for you to have surgery. This may depend on:

  • the type of APC gene variant you have
  • your age
  • your feelings about surgery and the risk of cancer
  • the results of your bowel screening tests.

The thought of surgery may be stressful or difficult to cope with. Although surgery is very effective at preventing bowel cancer, there are risks and side effects. For some people, there may be a risk that surgery will affect their fertility. Your surgeon will explain what to expect and the risks and benefits of your surgery.

Sometimes surgery can be delayed, as long as you do not have too many polyps. If surgery is delayed, it is important to have a colonoscopy at a specialist centre every 6 to 12 months.

If you have any questions or worries, talk to your surgeon, nurse or FAP specialist. Sometimes it helps to talk to other people in the same situation. We have more information about getting support at the end of this page.

Removing the colon

It may be possible to remove the colon and join the small bowel to the top of the rectum or the sigmoid (the section of bowel that joins the colon to the rectum). This operation is called an ileo-rectal or ileo-sigmoid anastomosis. 

After the operation, you will be able to go to the toilet normally, but will need to go more often. There is still a risk that polyps may develop in the rectum. You will need a test to check this every 6 to 12 months, depending on how many polyps are growing in the rectum.

Removing the colon and the rectum

Sometimes the lining of the rectum is also removed to avoid the risk of polyps forming there. If the rectum is removed, it may be possible to replace it with a pouch. This is made using a piece of the small bowel.

This is a complex operation and involves making a stoma. To make a stoma, the surgeon brings the end of the small bowel out on to the skin of the tummy (abdomen). You wear a bag over the small bowel opening. This collects stools (poo).

Having a stoma allows the joins that hold the pouch in place to heal. After about 3 months, you may have another operation to reverse the stoma. You will then be able to go to the toilet normally. But you will need to go more often than before. The stools you pass will be looser, and you might need to take anti-diarrhoea medicine. If the rectum has been removed, you will still need to have the pouch checked every year. This is because polyps can continue to grow there.

Sometimes the rectum is removed but it is not possible to replace it with an internal pouch. In this situation, you will have a permanent stoma.

It can take time to get used to having a stoma. There are specialist nurses who can help and support you through this. They are called stoma nurses.

Know your body

If you know your body and what is normal for you, it will help you be aware of any changes. If you notice a change in how you feel or how your body works, it is better to be safe and get it checked. Always see your GP or your FAP specialist if you have symptoms that are ongoing or unusual for you.

We have more information about the signs and symptoms of cancer that you may find helpful.

Health and lifestyle

Many people who find out they have a high cancer risk decide to improve their lifestyle in some way. Some people find a healthy lifestyle makes them feel they are doing everything they can to control their cancer risk.

It is not clear how much lifestyle factors affect cancer risk in people with FAP. But there is strong evidence a healthy lifestyle improves your general health and well-being. Ask your genetics specialist for advice about your risks. Your genetics specialist, GP or practice nurse can give you more information about healthy living.

Planning a family

If you have FAP, there is a 1 in 2 (50%) chance that any child you have could inherit it from you. You may have questions about this if you are planning to get pregnant or start a pregnancy. Your genetics specialist can give you more information. 

Sometimes tests and fertility treatments can be used to ensure a parent’s gene variant is not passed to a child. The Human Fertilisation & Embryology Authority (HFEA) has information about this.

Finding support

FAP can be difficult to cope with. There is the uncertainty of not knowing whether you will develop cancer. There are often complicated decisions to make about the right time to have surgery. 

Some people choose to have screening for longer, because they find the thought of surgery very difficult. Other people may choose to have surgery earlier because they find waiting for each screening hard. 

It is important to talk to your FAP specialist, GP or nurse. They will be happy to answer any questions you have.

There is no right or wrong way to cope. But help is there if you need it. If you are struggling, talking about your feelings and worries may help. You may want to talk to a partner, family member or friend. You can also talk to our cancer support specialists for free on 0808 808 0000. Or you can share your thoughts with members of our Online Community.

Here are some other organisations you may find helpful:

About our information

  • References

    Below is a sample of the sources used in our family history and cancer information. If you would like more information about the sources we use, please contact us at  cancerinformationteam@macmillan.org.uk

    Monahan KJ, Bradshaw N, Dolwani S, et al. Guidelines for the management of hereditary colorectal cancer from the British Society of Gastroenterology (BSG)/Association of Coloproctology of Great Britain and Ireland (ACPGBI)/United Kingdom Cancer Genetics Group (UKCGG). Gut. 2020; 69: 411–444. Available from DOI: 10.1136/gutjnl-2019-319915 (accessed October 2022).

    NICE. Familial breast cancer: classification, care and managing breast cancer and related risks in people with a family history of breast cancer. Clinical guideline [CG164]. Last updated: 20 November 2019. Available from https://www.nice.org.uk/guidance/CG164 (accessed October 2022).

    NICE. Molecular testing strategies for Lynch syndrome in people with colorectal cancer. Diagnostics guidance [DG27]. Published: 22 February 2017. Available from https://www.nice.org.uk/guidance/dg27 (accessed October 2022).

    NICE. Testing strategies for Lynch syndrome in people with endometrial cancer. Diagnostics guidance [DG42]. Published: 28 October 2020. Available from https://www.nice.org.uk/guidance/dg42 (accessed October 2022).

  • Reviewers

    This information has been written, revised and edited by Macmillan Cancer Support’s Cancer Information Development team. It has been reviewed by expert medical and health professionals and people living with cancer. It has been approved by Senior Medical Editor, Dr Marc Tischkowitz, Reader and Honorary Consultant Physician in Medical Genetics.

    Our cancer information has been awarded the PIF TICK. Created by the Patient Information Forum, this quality mark shows we meet PIF’s 10 criteria for trustworthy health information.

The language we use

We want everyone affected by cancer to feel our information is written for them.

We want our information to be as clear as possible. To do this, we try to:

  • use plain English
  • explain medical words
  • use short sentences
  • use illustrations to explain text
  • structure the information clearly
  • make sure important points are clear.

We use gender-inclusive language and talk to our readers as ‘you’ so that everyone feels included. Where clinically necessary we use the terms ‘men’ and ‘women’ or ‘male’ and ‘female’. For example, we do so when talking about parts of the body or mentioning statistics or research about who is affected.

You can read more about how we produce our information here.

Date reviewed

Reviewed: 01 October 2022
Next review: 01 October 2025
Trusted Information Creator - Patient Information Forum
Trusted Information Creator - Patient Information Forum

Our cancer information meets the PIF TICK quality mark.

This means it is easy to use, up-to-date and based on the latest evidence. Learn more about how we produce our information.