A randomized clinical trial was conducted by the European Organization for Research and Treatment for Cancer (EORTC) Gastrointestinal Cancer Cooperative Group to study the effectiveness of irradiation therapy administered in a dosage of 34.5 Gy, divided into 15 daily doses of 2.3 Gy each before radical surgery for rectal cancer (T2, T3, T4, NX, MO). Four hundred sixty-six patients were entered in the clinical trial between June 1976 and September 1981. Tolerance and side effects of preoperative irradiation were acceptable. The overall 5-year survival rates were similar in both groups. When considering only the 341 patients treated by surgery with a curative aim, the 5-year survival rates were 59.1% and 69.1% in the control group and in the combined modality group, respectively (p = 0.08). The local recurrence rates at 5 years were 30% and 15% in the control group and the adjuvant radiotherapy group, respectively (p = 0.003). Although this study did not show preoperative radiotherapy to have a statistically significant benefit on overall survival, it does have a clear effect on local control of rectal cancer. Therefore, before performing radical surgery, this adjuvant therapy should be administered to patients who have locally extended rectal cancer.