Previously documented and new patients with ulcerative colitis seen between 1975 and 1980 in the Gastro-intestinal Clinic of Groote Schuur Hospital were studied to establish the local incidence and clinical features of this disease. There were 220 patients and the mean follow-up was 7,7 +/- 0,4 years. Sixty per cent of patients were White, 37% Coloured and 3% Black. The incidence for the Coloured and White population was calculated to be 1,3 and 2,4/100 000 per year during 1970-1974 and 1,6 and 2,1/100 000 per year during 1975-1980, respectively. In Jews the rates were 8,5 and 10,4/100 000 per year for the two periods. Insufficient data are available to calculate an incidence for the Black population. The disease was limited to less than 15 cm above the anus in 14% of patients, to the rectosigmoid colon in 45%, and to the rest of the colon in 40%. Although the severity of symptoms was related to the extent of disease, 22% of patients with extensive colitis had mild symptoms, while 15% with disease limited to the rectum had severe symptoms. The clinical features were similar in the White and Coloured population groups. A total colectomy was performed on 20% of patients with extensive colitis; in 2% the disease was complicated by colonic carcinoma.