FAP is a rare condition that can run in families. Conditions that run in families are referred to as inherited or familial. People with FAP are very likely to develop bowel cancer if they don't have treatment. About 1 in every 100 of all bowel cancers is linked to FAP.
People with FAP have hundreds or thousands of small growths called polyps in their large bowel. These polyps are sometimes called adenomas. This is where the ‘adenomatous’ part of the name FAP comes from. Polyposis means lots of polyps.
FAP can also affect people who don't have a family history of it. This happens in about 1 in 3 people with FAP.
The polyps usually start to appear when a person is in their teens. If the polyps aren't treated, one or more of them will almost certainly develop into cancer (usually by the age of 40). There's a type of FAP called attenuated FAP where the polyps appear about ten years later than with the usual FAP.
Most people with FAP are offered the option of having an operation to remove the large bowel when they're young. This can be hard to cope with, but it means they can avoid getting bowel cancer.