The natural history of ulcerative colitis (UC) has been poorly described in children. In a geographically derived incidence cohort diagnosed from 1988 to 2002, we identified 113 UC patients (age 0-17 years at diagnosis) with a follow-up of at least 2 years. The cumulative risk of colectomy was estimated by the Kaplan-Meier method. Risk factors for disease extension were assessed with logistic regression models, and risk factors for colectomy with Cox hazards proportional models. Median follow-up time was 77 months (46-125). At diagnosis, 28% of patients had proctitis, 35% left-sided colitis, and 37% extensive colitis. Disease course was characterized by disease extension in 49% of patients. A delay in diagnosis of more than 6 months and a family history of inflammatory bowel disease were associated with an increased risk of disease extension, with odds ratios of 5.0 (1.2-21.5) and 11.8 (1.3-111.3), respectively. The cumulative rate of colectomy was 8% at 1 year, 15% at 3 years, and 20% at 5 years. The presence of extra-intestinal manifestations (EIMS) at diagnosis was associated with an increased risk of colectomy (hazard ratio (HR)=3.5 (1.2-10.5)). Among the patients with limited disease at diagnosis, the risk of colectomy was higher in those who experienced disease extension than in those who did not (HR=13.3 1.7-101.7). Pediatric UC was characterized by widespread localization at diagnosis and a high rate of disease extension. Twenty percent of children had their colon removed after 5 years. The colectomy rate was influenced by disease extension and was associated with the presence of EIMS at diagnosis.