Patients with pancolonic chronic ulcerative colitis are at increased risk to develop carcinoma of the colon. Controversy continues, however, as to whether this carcinoma is more "virulent" than "type ordinaire" carcinoma of the colon and as to the best way to manage these patients. This study reviews the characteristics and survival of 70 patients with cancer of the colon superimposed on chronic ulcerative colitis. Patients with carcinoma identified incidentally during prophylactic colectomy for chronic ulcerative colitis fared well (5-year survival of 72 per cent), while those with clinical symptoms or radiographic suggestion of cancer had a poor survival rate (5-year survival of 35 per cent). Patients with panproctocolitis, 10 years of disease, and early onset of disease are most likely to have cancer superimposed on chronic ulcerative colitis. This cancer is likely to have a poorer prognosis than type ordinaire cancer of the colon. Prophylactic proctocolectomy should be considered before evidence suggesting carcinoma develops.