What is chronic myeloid leukaemia (CML)?

Chronic myeloid leukaemia (also called CML or CML leukemia) is a cancer of the blood cells.

About 750 people in the UK are diagnosed with CML each year. CML can affect people at any age, but it is more common as people get older. It usually develops very slowly. For many people, CML can be well controlled, and it will not shorten their life.

We have separate information about other types of leukaemia.

How chronic myeloid leukaemia develops

Blood cells are made in the bone marrow. The bone marrow is the spongy material found inside our bones. The bone marrow usually makes billions of new blood cells every day to replace old and worn-out blood cells.

All blood cells are made from blood stem cells. These are blood cells at the earliest stage of their development (called blast cells). There are two types of blood stem cell:

  • lymphoid stem cells – which make a type of white blood cell called lymphocytes
  • myeloid stem cells – which make the other white blood cells such as neutrophils, red blood cells and platelets.


MACD190 Blood cells labelled
Image: How blood cells divide


CML develops when some white blood cells start behaving abnormally.

Symptoms of chronic myeloid leukaemia (CML)

CML develops slowly and many people do not have symptoms in the early stages. Sometimes CML is found by chance when you have a blood test before an operation, or as part of a routine health check.

In the early stages of CML, any symptoms are usually mild and develop slowly. They can be confused with the symptoms of more common illnesses, such as flu.

We have more information about general symptoms of leukaemia.

Causes and risk factors of chronic myeloid leukaemia (CML)

It is not clear why people get CML. It is not linked to smoking, diet, exposure to chemicals or infections. It also does not run in families. Like other cancers, CML is not infectious and cannot be passed on to other people.

There are some factors that might increase the risk of developing CML.

  • Age

    CML can develop at any age. But it is more common as people get older. Over half of people diagnosed with CML in the UK are over the age of 60.

  • Sex

    CML is slightly more common in men than women, but the reason for this is not known.

  • Radiation exposure

    Exposure to very high radiation levels increases the risk of developing CML. For example, these might be high levels of radiation following a nuclear accident. Very few people in the UK will be exposed to radiation levels high enough to increase their risk of CML.

    Research has found no link between the risk of developing CML and:

    • living near nuclear power stations
    • exposure to electro-magnetic fields
    • living near high-voltage electricity cables
    • household radon (naturally occurring gas).

How chronic myeloid leukaemia (CML) is diagnosed

If your GP thinks you may have CML, they will refer you to a haematologist for further tests. A haematologist is a doctor who specialises in diagnosing and treating blood problems.

The haematologist will ask you about any illnesses or health problems you have had. They will examine you to check if your spleen is enlarged.

Tests for CML may include:

  • Blood tests

    You will have blood tests at the hospital. These tests will:

    • check the numbers of blood cells in your blood, called a full blood count (FBC)
    • look for leukaemia cells.

    If there are leukaemia cells in your blood, the haematologist will arrange more tests for you. These will find what type of leukaemia you have and what phase it is. This will help your doctors plan your treatment.

  • Bone marrow test

    A bone marrow test is when a doctor or nurse takes a small sample of bone marrow to test for abnormal white blood cells. They usually take the bone marrow from the back of the hip bone (pelvis). Rarely, they may take it from the breast bone (sternum).

  • Cytogenetic and molecular tests

    These tests look for gene changes (mutations) inside the leukaemia cells. Tests may include:

    • Philadelphia chromosome test – doctors use the blood and bone marrow samples they have taken to look for the Philadelphia chromosome.
    • Polymerase chain reaction (PCR) test – this blood test looks for the BCR-ABL1 gene in the leukaemia cells. It is a very sensitive test, which can detect tiny amounts of leukaemia.

    If you are diagnosed with CML, you will have regular PCR blood tests. Doctors use this test to check how well treatment is working.

  • Ultrasound scan

    You might have an ultrasound scan to check the size of your spleen and liver. Ultrasound scans use sound waves to build up a picture of the inside of the body. This is a painless test that only takes around 10 to 15 minutes.

Phases of chronic myeloid leukaemia (CML)

Doctors know the phase of CML from:

  • the number of immature blood cells (blast cells) in your the blood and bone marrow
  • the symptoms you have.

Most people are diagnosed when CML is in the chronic phase.

CML usually develops slowly. There are three phases of CML.

  • Chronic phase

    When CML is in the chronic phase, there may be no symptoms and most people can have a normal life. Treatment is with tablets you can take at home. You will have regular blood tests to check how well the treatment is working.

    For most people, the leukaemia can be well controlled for as long as they continue to take treatment.

    In the chronic phase, less than 15 in 100 blood cells in the blood or bone marrow (15%) are blast cells.

  • Accelerated phase

    In a small number of people, the leukaemia may progress from the chronic phase to the accelerated phase. This can happen if the CML does not respond well to treatment. Sometimes people are diagnosed with CML in the accelerated phase.

    In the accelerated phase, less than 2 in 10 blood cells in the blood or bone marrow (10 to 19%) are blast cells.

    In this phase, there are more blast cells in the blood or bone marrow. You may also develop symptoms such as:

    • tiredness
    • weight loss
    • bone pain
    • sweating and a high temperature at night.

    If you feel unwell or develop new symptoms, tell your doctor.

  • Blast phase

    In some people, CML in the accelerated phase may transform into the blast phase. Rarely, people are diagnosed with CML in the blast phase. Or, the leukaemia progresses straight to the blast phase from the chronic phase.

    The blast phase is like an acute leukaemia. In this phase, more than 2 in 10 blood cells in the blood or bone marrow (20%) are blast cells. The blast phase is sometimes called blast crisis.

  • Relapse

    Relapse means the leukaemia cells have come back after a time in remission (where there are no signs of leukaemia cells). In CML, this is usually found using the PCR test.

    Most people are diagnosed when CML is in the chronic phase. Doctors know the phase of CML from:

    • the number of immature blood cells (blast cells) in your blood and bone marrow
    • your symptoms.

Treatment of chronic myeloid leukaemia (CML)

A team of specialists will meet to discuss the best possible treatment for you. This is called a multidisciplinary team (MDT).

Your cancer doctor or specialist nurse will explain the different treatments and their side effects. They will also talk to you about certain things to think about when making treatment decisions.

The aim of treatment is to put CML into remission (where there are no signs of leukaemia cells). Treatments used can include the following:

  • Tyrosine kinase inhibitors (TKIs)

    The main treatment for CML is with drugs called tyrosine kinase inhibitors (TKIs).

  • Chemotherapy

    Chemotherapy uses anti-cancer (cytotoxic) drugs to destroy or damage leukaemia cells. Chemotherapy is only sometimes used for CML. 

  • Stem cell transplant

    If CML is not controlled by TKI’s or if you are diagnosed in the blast phase, your doctors may suggest chemotherapy and a stem cell transplant.

  • Interferon alpha

    If other treatments have not worked, some people may be given interferon alpha in the chronic phase of CML. Doctors may also use it for women who need treatment and are pregnant  or want to become pregnant.

  • Leukapheresis

    You might have a very high number of white blood cells in your blood when you are diagnosed with CML. Doctors can treat this using a process is called leukapheresis.

  • Clinical trials

    You may be invited to take part in a clinical trial of a new treatment for CML.

We have more information about treatment for CML.

Fertility and chronic myeloid leukaemia (CML) treatment

Some cancer treatments can affect fertility or harm a developing baby. Because of this you may be advised to use contraception to prevent a pregnancy. If you want to have children or think you may in the future, talk to your doctor about this as soon as possible. They can talk to you about the possible options for planning your treatment.

We have more information about fertility for men and fertility for women.

Living with chronic myeloid leukaemia (CML)

Follow-up and monitoring CML

If you have treatment with a TKI, you will be monitored by your healthcare team every 1 to 2 weeks.

At these check-ups, your doctor will:

  • ask about your general health
  • ask about any new symptoms or side effects of treatment
  • do blood tests to check the numbers of blood cells (FBC) and leukaemia cells (PCR test).

Sometimes they may take a bone marrow sample. Your doctor can tell you how often you might need this.

These test results help your doctors know how well the treatment is working to control the leukaemia. They will also check for any side effects. They can make any changes if needed.

As time goes on, you will not need to see your doctors as often. Eventually, you may only need a check-up every 3 to 6 months.

If you have another type of treatment, your doctor or nurse will tell you how often you will have appointments.


Most people with CML live a normal lifespan. To help you stay as well as possible, you may want to make changes to your lifestyle. Even if you had a healthy lifestyle before your diagnosis, you may want to make the most of your health.

A healthy lifestyle does not have to be difficult or expensive. It is about making small changes to the way you live. This will improve your health and sense of well-being. It will also lower your risk of getting other illnesses and some other cancers.

Understanding more about CML and its treatment can also help you cope. This means you can discuss treatment, tests and check-ups with your doctors and nurses. It also means you can be involved in making decisions. This can make you feel more confident and more in control. Here are some things to consider:

  • Get involved in your healthcare

    This includes taking your medicines as prescribed and always going to your hospital appointments. If you have any problems or notice any new symptoms between your appointments, tell your doctor as soon as possible.

  • Self-help and support groups

    Talking about your feelings can help reduce stress, anxiety and isolation. Self-help or support groups offer a chance to talk to other people who may be in a similar situation and facing the same challenges as you.

  • Online support

    There are online support groups, social networking sites, forums, chat rooms and blogs for people affected by leukaemia. You can use these to ask questions and share your experience. You can also visit our Online Community.

Getting support

Everyone has their own way of dealing with illness and the different emotions they experience. You may find it helpful to talk things over with family and friends or your doctor or nurse. 

The organisations below offer information and support:

  • Anthony Nolan
    Anthony Nolan is the UK’s largest stem cell and bone marrow register.
  • Leukaemia CARE
    Leukaemia CARE provides care and support to patients, their families and carers whose lives have been affected by leukaemia, lymphoma or a related blood disorder.
  • Blood Cancer UK
    Blood Cancer UK is a blood cancer research charity that provides information and support on any type of blood cancer.

Macmillan is also here to support you. If you would like to talk, you can:

About our information

  • References

    Below is a sample of the sources used in our chronic myeloid leukaemia (CML). If you would like more information about the sources we use, please contact us at cancerinformationteam@macmillan.org.uk

    Smith G, Apperley J, Milojkovic D, et al. A British Society for Haematology Guideline on the diagnosis and management of chronic myeloid leukaemia. British Journal of Haematology, 2020; 191, 2, Available from https://b-s-h.org.uk/ [accessed on October 2020].

    Hochhaus A, Saussele S, Rosti G, et al. Chronic Myeloid Leukaemia: ESMO Clinical Practice Guidelines. Annals of Oncology, 2017; 28 (suppl 4), iv41-iv51. Available from https://www.esmo.org/ [accessed on October 2020].

  • 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 Anne Parker, Consultant Haematologist.

    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 December 2020
Next review: 01 December 2023

This content is currently being reviewed. New information will be coming soon.

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.