Carcinoma of the colon that arises in patients with Crohn's disease is being reported with increasing frequency. To help clarify the nature of this association, records of 25 patients with Crohn's disease and colorectal carcinoma seen from 1957 through 1989 were reviewed. One patient had leiomyosarcoma of the rectum, and two patients had the onset of Crohn's disease after the diagnosis and treatment of colorectal carcinoma. Therefore, 22 patients were available for complete retrospective analysis. The median age at diagnosis of Crohn's disease was 37 years (range, 15-67 years), and the median age at diagnosis of carcinoma was 54.5 years (range, 32-76 years). The median duration of symptoms preceding the discovery of colorectal carcinoma was 18.5 years (range, 0-32 years). Carcinoma arose in colonic segments with known Crohn's disease in 77 percent of patients, and six patients (27 percent) had associated colonic mucosal dysplasia. One lesion was classified as Dukes A, nine lesions were Dukes B, five lesions were Dukes C, and seven lesions were Dukes D. Patients with an onset of Crohn's disease before the age of 40 years had primarily Dukes C or D lesions and consequently poor survival. Most patients presented with nonspecific signs and symptoms, with nothing to distinguish the activity of the Crohn's disease from the presence of colorectal neoplasm. Younger patients with long-standing Crohn's disease should be considered for colonic surveillance to permit earlier diagnosis and treatment of potential colorectal carcinoma.