A regional inception cohort of 1161 ulcerative colitis patients was followed from diagnosis to the end of 1987. Follow-up rate for death and occurrence of cancer was 99.9%. Median observation time 11.7 years, range 0-26 years. One hundred and forty-one deaths were observed, 26 due to ulcerative colitis or complications thereof. No significant excess mortality was found after the first year, but in the year of diagnosis the relative risk of death was 2.4 (p < 0.001). The cumulative colectomy rate was 32.4% 25 years after diagnosis. The initial extent of disease significantly influenced the colectomy probability, being 35% in total colitis, 19% in substantial colitis and 9% in distal colitis within the first five years after diagnosis. Six patients developed colorectal cancer within the observation period. Compared to the expected number of 6.6 the relative risk for patients with ulcerative colitis was 0.9. The calculated cumulative cancer incidence was 3.1% after 25 years (95% confidence limits 0.0-6.8). The calculated lifetime risk (0 to 74 years) for development of colorectal cancer was 3.5% for the patients compared to 3.7% for the Danish population. We conclude that with an active approach to medical and surgical treatment as was practiced here, those patients who are left with their colon intact bear no significantly increased risk of colo-rectal malignancy.