From 1955 through 1974, 336 patients with ulcerative colitis diagnosed before age 21 years were studied. In 93 patients (29%), a blood relative had ulcerative colitis, one case of Crohn's disease being found. The total colon was involved in 63% of patients; the entire colon or all but the rectal stump was removed in 35%. Eighteen patients died, nine of carcinoma of the colon. Sixty-five percent of patients had symptoms for longer than 6 months before the diagnosis of ulcerative colitis. If the diagnosis was delayed more than 24 months, there was a statistically significant correlation with increased rate of operations and complications and less good quality of life. When the 20-year study period was divided into two 10-year periods, the operative and complication rates were significantly different. Early diagnosis and treatment appear to improve the long-term prognosis of young patients with ulcerative colitis.