The incidence of colorectal cancer was studied by the actuarial method in 959 patients with idiopathic proctocolitis seen from 1942 to 1981. Forty five per cent had rectal, 23% left-sided, and 32% total involvement of the colon. Six cancers were found: one in the rectal, one in the left-sided, and four in the total form of the disease. The risk of cancer per patient year in total colitis was zero per 2151 patient years in the first decade, 1/462 in the second decade, 1/315 in the third decade, and 1/75 in the fourth decade. The cumulative risk of developing cancer was zero at 10 years of duration of the disease, approximately 5% at 20 years, 15% at 30 years, and 20% at 35 years. This increase in risk of cancer is less than reported in some other series. Geographical differences in the incidence of cancer in proctocolitis could influence the risk and therefore also the long-term management of patients with proctocolitis in different geographical areas.