An estimated 44,000 cases of rectal carcinoma arise annually in the United States. The traditional management of this disease has been surgery alone, but advances in adjuvant therapy offer potential for improvement of local control, disease-free survival, and survival. In the last two decades, many multicenter randomized trials of adjuvant preoperative and postoperative radiation therapy for rectal carcinoma have been reported. The design and results of these trials are critically reviewed. Results from preoperative trials have been conflicting, reflecting the heterogeneity of the trial designs. Large postoperative adjuvant trials have been reported recently. The combined analysis of local recurrence data from the mature, published trials indicates that radiation therapy results in improved local control (P = 0.02), an important concern in rectal carcinoma as local recurrences present vexing and painful clinical problems often refractory to conventional management. These trials also have shown that radiation therapy can contribute to improved survival in the combined modality setting. Improvements in the clinical outcome of rectal cancer should be possible with appropriate adjuvant therapy. The success of combined modality adjuvant therapy for rectal carcinoma may serve as a model to aid in the design of therapeutic regimens for other solid tumors.