An inherited condition in which numerous polyps form in the large intestine (colon). In familial polyposis, the polyps begin as precancerous adenomas, usually during puberty, but often become cancerous by age 40. Harmless cysts may appear on the skin of the scalp, face, arms, or legs. Bony growths in the jaw or on the skull sometimes appear. Other symptoms include blood or mucus in the stool, diarrhea, and occasional abdominal cramps, with or without weight loss. But by the time these symptoms develop, cancer may already be present, so it is particularly important that people with a family history of familial polyposis be examined for the condition early in childhood or adolescence.