Colonic lymphoma is a rare complication of ulcerative colitis. Two cases are described in patients who had had ulcerative colitis for 12 and 22 years respectively. Both patients presented with a recent change in their symptoms, which had become increasingly severe and which had not remitted with customary treatment for ulcerative colitis. Physical and haematological examinations revealed no evidence of generalized lymphoma, though barium enema studies indicated the sites of the lymphomatous lesions superimposed on chronic ulcerative colitis which were confirmed at operation and biopsy.