Between July 1975 and July 1986, 112 patients with adenocarcinoma of the rectum were treated using preoperative irradiation followed by excisional surgery on the colorectal surgery service of Jewish Hospital at Washington University Medical Center in St. Louis. There were 68 men and 44 women in this study, with ages ranging from 19 to 94 years of age. In all cases, the rectal cancers were believed to be transmurally invasive based on initial clinical examination. Included in this group were 13 patients with poorly differentiated tumors and 51 patients with tumors fixed to surrounding tissues. Between 1975 and 1980, we used 2000 cGy preoperative irradiation followed by immediate excisional surgery to treat 22 patients. Excisional surgery for cure was divided between abdomino-perineal resection of the rectosigmoid in eleven patients, low anterior resection of the rectosigmoid in eight patients, and a low Hartmann's procedure in three patients. Five-year survival for 20 patients with potentially curable lesions (Dukes' A, B, and C), was 85%, and there was no local recurrence. Between 1980 and 1986, 90 patients were treated with 4500 cGy preoperative irradiation over a 5-week period followed by a 6-week waiting period, before excisional surgery. There were 72 patients with Dukes' A, B, and C lesions. Fifty patients underwent abdomino-perineal resection of the rectosigmoid, 33 patients underwent low anterior resection of the rectum, and seven patients underwent a low Hartmann's procedure. Five-year survival was 86%. Local recurrence was 1.8%. Tumor fixation and histologic dedifferentiation were the only factors that influenced survival. Five-year survival of patients with fixed poorly differentiated tumors was 27% as compared to 87% in patients with nonfixed well-differentiated tumors (p < 0.0001). Tumor fixation was not a significant factor in itself. Preoperative external beam irradiation improves survival, local control, and resectability in patients with rectal cancer. This effect may be due to the treatment of the 'tangential' margins and local lymph node metastases. Preoperative staging can be accomplished by determining fixation and differentiation of the tumor when preoperative irradiation is used.