Of 213 patients with proctitis of all etiologies seen between 1977 and 1987, we studied an original cohort of 96 patients with idiopathic ulcerative proctitis (mean follow-up of 62 months). The diagnosis was made according to strict inclusion criteria, and all cases with any initial feature suggestive of Crohn's disease were excluded. Follow-up showed that 13 patients (13.6%) eventually progressed to Crohn's disease, usually within the first 3 years of the initial diagnosis of idiopathic ulcerative proctitis. Their clinical, endoscopic and histological features at initial presentation were indistinguishable from those in whom the diagnosis remained that of idiopathic ulcerative proctis. In nine of 13 of these patients (70.0%), the clinical course was characterized by a more protracted course and a poorer response to standard treatment. Such features were not found in those in whom the diagnosis of idiopathic ulcerative proctitis was maintained. Clinicians should be aware that Crohn's disease may present intially as apparent idiopathic ulcerative proctitis.