Between 1977 and 1984, 17 patients received external beam irradiation after subtotal resection of rectal carcinoma. Ten patients had microscopic residual disease and 7 had gross residual disease. In the group with microscopic residual disease, 4 had tumor cut through with pathologically involved margins, 5 had adjacent unresected structures that were biopsy positive, and 1 had tumor spillage into the pelvis. The patients with gross residual disease were noted by the surgeon to have visible tumor after maximal debulking. Nine of 17 cases had involved pelvic lymph nodes. Radiation was administered to the pelvis with 4, 6, or 10 MV photons. Doses ranged from 40 to 60 Gy, with a median dose of 50 Gy given at 1.8 to 2.0 Gy per fraction, 5 days per week. Three patients received bacillus Calmette-Guérin (BCG), 2 received 5-fluorouracil (5-FU), and 1 received hycanthone. Thirteen of the 17 patients (76%) experienced local failure and, of these, 10 also developed distant disease. No patients developed distant metastasis in the absence of local failure. Local control was achieved in 3 of 10 patients (30%) with microscopic residual and 1 of 7 (14%) with gross residual. Four of the 17 patients (24%) have remained free of disease for greater than 5 years. External beam irradiation is capable of producing long-term survival and local control in a minority of patients with rectal cancer after subtotal resection. Investigation of more aggressive forms of therapy such as the addition of intraoperative irradiation, brachytherapy, radiation dose modifiers, and chemotherapy is warranted.