All 1274 patients in Stockholm County who were diagnosed as having ulcerative colitis during the 25-year period 1955-79 were followed up and studied with regard to prognosis and mortality. There were 109 deaths recorded--41 caused by ulcerative colitis and 68 from other causes. The cumulative survival probability (computed by life-table methods) was worse than expected. For males it was approximately 80% at 20 years, to be compared with the approximately 97% expected in the background population. For females the corresponding figures were approximately 85% and 98%, respectively. The excess death risk increased with increasing age and was more commonly due to unrelated causes than to ulcerative colitis as such. In patients with total colitis at onset the death risk was greater than for other patients during the first years of disease. Thereafter the prognosis was, if anything, better than in other cases. There was no major change in postoperative mortality or general mortality pattern during the 25 years of observation.