OBJECTIVE This study was designed to evaluate functional outcome in patients treated with preoperative radiotherapy after low anterior resection and a coloanal anastomosis for low rectal cancer. METHODS Functional outcome data from patients enrolled in a prospective randomized trial comparing 3 reconstructive procedures were evaluated with respect to administration of preoperative radiotherapy. Incontinence was assessed with a questionnaire on bowel function including the Fecal Incontinence Severity Index; sexual function was assessed with the Sexual Health Inventory for Men and a gender-specific questionnaire for women. Quality of life was assessed with SF-36 scores. RESULTS Of 364 patients enrolled, 153 (42%) had no radiotherapy or chemotherapy, and 211 (58%) had preoperative radiotherapy; 186 (51%) had chemotherapy in addition to radiotherapy. Comparison of irradiated vs. nonirradiated patients showed no significant differences in postoperative morbidity (29.9% vs. 35.3%; P = 0.27). Two-year follow-up of 297 patients showed greater impairment of bowel function in irradiated patients (n = 170) vs. nonirradiated patients (n = 127): e.g., mean number of daily bowel movements at 12 months, 4.2 +/- 3.5 vs. 3.5 +/- 2.6, P = 0.032; urgency, 85% vs. 67%, P = 0.002). Antidiarrheal use was significantly higher in irradiated patients vs. nonirradiated patients at 4 (P = 0.043), 12 (P = 0.002), and 24 (P = 0.001) months. Sexual Health Inventory for Men scores indicated poorer function in irradiated patients at 24 months (P = 0.039). Preoperative radiotherapy had no deleterious effects on quality of life. Multivariate analyses showed that negative effects of preoperative radiotherapy on urgency at 4 months (P = 0.002) and antidiarrheal use at 24 months were independent of reconstruction technique, but a positive effect of reconstruction with a J-pouch was still observed in patients who received radiotherapy. CONCLUSIONS Preoperative radiotherapy does not increase overall morbidity but is associated with poorer functional outcome after low anterior resection with coloanal anastomosis. Preoperative radiotherapy and the J-pouch are nonconfounding predictors of functional outcome up to 24 months after surgery.