PURPOSE:Preoperative radiotherapy for rectal cancer avoids radiation to the reconstructed rectum and may circumvent the detrimental effects on bowel function associated with postoperative radiotherapy. We compared the long‐term functional results of patients who received preoperative radiotherapy, postoperative radiotherapy, or no radiotherapy in conjunction with low anterior resection and coloanal anastomosis to assess the impact of pelvic radiation on anorectal function.METHODS:One hundred nine patients treated by low anterior resection and straight coloanal anastomosis for rectal cancer between 1986 and 1997 were assessed with a standardized questionnaire at two to eight years after resection. All radiotherapy was given to a total dose of 4,500 to 5,400 cGy with conventional doses and techniques. Most patients received concurrent 5‐fluorouracil‐based chemotherapy.RESULTS:There were 39 patients in the preoperative radiotherapy group, 11 patients in the postoperative radiotherapy group, and 59 patients in the no radiotherapy group. The postoperative radiotherapy group reported a significantly greater number of bowel movements per 24‐hour period (P< 0.01) and significantly more episodes of clustered bowel movements (P< 0.02) than either the preoperative radiotherapy group or the no radiotherapy group. No significant difference in anal continence or satisfaction with bowel function was found among the three groups.CONCLUSION:In this study of straight (nonreservoir) coloanal anastomoses, postoperative pelvic radiotherapy had significant adverse effects on anorectal function, with higher rates of clustering and frequency of defecation than with preoperative radiotherapy. No differences in continence rates were demonstrated, perhaps because of the sample size of the compared groups. We attribute the adverse effects of postoperative radiotherapy to irradiation of the neorectum, which is spared when treatment is given preoperatively. The deleterious effects of adjuvant radiation on long‐term anorectal function can be reduced by preoperative treatment.