BACKGROUND From 1980 to 1987, 849 patients with clinically resectable rectal adenocarcinoma were randomized into a controlled clinical trial to evaluate the role of preoperative radiotherapy. METHODS Patients were given either 25 Gy during 5 to 7 days before surgery or underwent surgery alone. RESULTS At a median follow-up time of 107 months (range, 62-144 months) the incidence of pelvic recurrence among 684 "curatively" operated patients was significantly lower among those who also received radiotherapy (P < 0.001) in all Dukes' stages. No significant difference was observed between the treatment groups with regard to frequency of distant metastases or overall survival. The time to local recurrence or distant metastasis and survival was significantly prolonged in the irradiated group. However, the postoperative mortality was 8% in the radiotherapy group compared with 2% in the surgery only group (P = 0.01). CONCLUSIONS Preoperative short term radiotherapy reduced the incidence of pelvic recurrences and prolonged survival related to rectal cancer compared with surgery alone. The postoperative morbidity was significantly higher in the irradiated group.