Pre or postoperative pelvic irradiation has demonstrated a definitive efficacy in reducing the local failure rate of rectal cancer treated with surgery alone. However it can induce late small bowel morbidity that could alter the therapeutic ratio. The clinical pictures of radiation enteritis include obstruction and diarrhea/malabsorption. Prognostic factors that increase the risk of late small bowel complications include extended fields out of the pelvis, irradiation dose, inappropriate irradiation technique, and increased small bowel irradiated volumes. The addition of chemotherapy increases acute but not late toxicity. Recommendations concerning the clinical practice are described. Radiotherapy may also alter the residual sphincter function and we recommend to assess correctly these complications.