A regional inception cohort of 1161 ulcerative colitis (UC) patients was followed up from diagnosis to the end of 1987. The follow-up rate for death and occurrence of cancer was 99.9% (median observation time, 11.7 years; range, 0-26 years). One hundred forty-one deaths were observed, 26 caused by UC 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 25 years after diagnosis was 32.4%. 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 5 years after diagnosis. Six patients developed colorectal cancer within the observation period. Compared with the expected number of 6.6, the relative risk for patients with UC 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 (0-74 years) for development of colorectal cancer was 3.5% for UC patients compared with 3.7% for the Danish population. It is concluded that with an active approach to medical and surgical treatment, as practiced here, patients whose colons are left intact bear no significantly increased risk of colorectal malignancy.