All patients with onset of ulcerative colitis before the age of 20 years from a population of 65,000 inhabitants were studied. The study period was 30 years (1961 through 1990). There were 32 patients, 18 male and 14 female, giving an incidence of 0.7 patients/100,000/year. Twenty-four patients had total colitis and 8 had limited disease. The observed duration of the disease was 0-30 (mean 18, median 20) years. One patient died of fulminant disease. No colorectal carcinoma was diagnosed. In 7 patients, low-grade dysplasia was diagnosed, which did not progress during an observation period of 2-10 years. DNA aneuploidy was detected in 6 patients. Seven patients (22%) were operated on after a disease duration of 0-10 (mean 4, median 3) years on clinical grounds. No cancer-prophylactic colectomy was performed. It is concluded that, in an unselected population of patients with early onset of ulcerative colitis, mortality and cancer risk are low. The need for operative treatment during the first 15 years of the disease was around 22 per cent. Endoscopic surveillance was given preference over prophylactic colectomy in patients with low-activity or quiescent disease.