A retrospective study of 267 patients with ulcerative colitis admitted to The Mount Sinai Hospital during the period 1960--1976 revealed 26 (9.7%) with adenocarcinoma of the colon. Twenty-one cases of colorectal cancer were observed among 158 patients with universal colitis (13%), and 5 occurred among 109 patients with left-sided disease (5%). Patients with left-sided disease tended to develop cancer at least a decade later than patients with universal disease. The median duration from onset of colitis to diagnosis of cancer was 20 yr for those with universal colitis, and 32 yr for those with left-sided colitis. The decade incidence of colorectal carcinoma increased from 0.4% in the first decade to 7.4% in the second, 15.9% in the third, and 52.6% in the fourth decade of follow-up. The estimated cumulative probability of developing cancer reached 34% at 30 yr and 64% at 40 yr. Cancer risk was positively correlated with duration and anatomic extent of colitis, but did not appear to be increased by early age at onset of disease.