To determine the risk and predictive factors for colonic extension in patients with ileal Crohn's disease.One hundred and fifty patients with ileal Crohn's disease and no specific colonic lesions on initial colonoscopy were studied retrospectively (median follow-up: 51 months).Twelve patients (8%) developed colonic lesions. Ten-year cumulated risks (95% confidence interval) for colonic extension were 17.2% (range: 5.8-28.6) in the whole group, and 22.4% (range: 8.7-36.1) in the group of 86 patients with repeated colonoscopy. Young age at diagnosis was the only factor predicting colonic extension. Seven patients with colonic extension required immunosuppressive therapy but none underwent surgery.Ileal Crohn's disease has a low tendency for colonic extension. Colonic extension has no major prognostic implications.