The ability to demonstrate an improvement in therapeutic ratio is critical in assessing new treatment modalities; an evaluation of treatment complications is essential for this purpose. We have studied the severe complications occurring after treatment with intraoperative radiation therapy (IORT) in patients with locally advanced carcinoma of the rectum. Four groups of patients were compared: Group 1 (80 patients) had treatment with surgery alone for mobile and resectable tumors; Group 2 (23 patients) had treatment with high dose preoperative irradiation followed by surgical resection for tumors which were fixed to adjacent structures and initially unresectable for cure; Group 3 (24 patients, primary disease) and Group 4 (17 patients, locally recurrent disease) had locally advanced tumors as in Group 2 but were treated with IORT after preoperative irradiation and attempted surgical resection. All but 3 complications occurred within one year of therapy. Severe complications were seen in 16% of patients in Group 1, 35% in Group 2, 21% in Group 3 and 47% in Group 4 (32% in Groups 3 and 4 combined). There was a statistically insignificant increase (p = .10) in the complication rate in all irradiated patients (locally advanced tumors) compared to surgery alone (clinically mobile tumors). These data indicate no increase in severe complications with the use of IORT. If the ongoing studies continue to show improved local control with the use of IORT, expanded use of this modality may be warranted.