A retrospective study was performed on 89 patients who underwent total colectomy and ileorectal anastomosis for extensive mucosal ulcerative colitis between the years 1957 and 1977 in order to determine the risk of developing cancer of the rectum. The 30-day operative mortality rate was 0%. Of the 84 patients available for follow-up study, four patients, (4.8%) developed a carcinoma of the rectum. The risk of cancer per patient-year was zero in the first decade, 1/206 in the second decade, and 1/116 in the third decade. The cumulative risk of developing cancer was 0% at 10 years, 2.1% +/- 2.1% at 15 years, 5.0% +/- 3.5% at 20 years, and 12.9% +/- 8.3% after 25 years of disease. Patients with cancer or precancer in the colon at the time of colectomy appear to be at high risk for the later development of rectal cancer.