We have examined the age at onset of both ulcerative colitis and colitis-associated colorectal cancer in 100 patients seen at Mount Sinai Hospital between 1959 and 1988. There were 85 patients with extensive colitis and 15 with left sided colitis. There was a strong direct correlation between the age at onset of ulcerative colitis and age at diagnosis of cancer (p less than 0.0001); this correlation was found both in patients with extensive colitis (p less than 0.0001) and in those with left sided colitis (p less than 0.005). Patients with left sided colitis developed both their colitis and their cancers about a decade later than did those with extensive disease, but the mean duration of colitis before diagnosis of cancer was virtually the same (about 21 years) in both groups, irrespective of the age at onset of disease.