The case notes of all patients treated for ulcerative colitis by colectomy and ileorectal anastomosis at the Gordon Hospital under the care of Mr S. O. Aylett from 1952 to 1976 have been reviewed. Three hundred and seventy-four patients left hospital with an ileorectal anastomosis and have been followed for periods up to 23 years. Twenty-two patients are known to have developed a carcinoma of the rectum. Within 10 years of the onset of the disease, no rectal carcinoma was found in 3534 patient-years. The risk was 1 in 185 patient-years between the tenth and twentieth years and 1 in 115 patient-years between the twentieth and thirtieth years. The cumulative risk was 6 per cent (+/- 2 per cent) at 20 years and 15 per cent (+/- 4 per cent) at 30 years. The need for meticulous follow-up is emphasized. The finding of dysplasia in rectal mucosal biopsies will, it is hoped, identify the patient at particularly high risk and enable rectal excision to be undertaken before carcinoma develops.