In a series of 1439 patients with ulcerative colitis, managed by one of the authors (E.S.R.H.), surgical resection was performed on 374 patients (26 per cent): colectomy, 273 (subtotal colectomy and mucous fistula, 172, colectomy and primary ileorectal anastomosis, 101); proctocolectomy, 61; and miscellaneous procedures, 40. Of the 172 patients undergoing subtotal colectomy and mucous fistula, 93 (54 per cent) subsequently required rectal excision, 33 (19 per cent) had ileorectal anastomosis performed as a second procedure, and in 46 (27 per cent) subsequently developed as a mucous fistula. Two hundred seventy-three patients were at risk for the development of rectal, cancer after subtotal colectomy; ten patients (3.6 per cent) subsequently developed rectal cancer. The cumulative probability of developing rectal cancer after subtotal colectomy reached 17 per cent at 27 years from disease onset. The tumors were more advanced in stage and of higher grade malignancy than those of a parallel general series of patients with rectal cancer uncomplicated by inflammatory bowel disease. Colectomy and ileorectal anastomosis has been successful for most patients. However, the experience of this series highlights the danger of carcinomatous transformation in the retained rectum, the requirement for regular long-term follow-up, the need for markers of precancerous change, and the value, where relevant, of prophylactic proctectomy.