HYPOTHESIS: Pelvic irradiation adversely affects anal sphincter function after proctectomy with coloanal anastomosis for low rectal and middle rectal (<10 cm from the anal verge) tumors. DESIGN: Case-control study. SETTING: Private, tertiary care referral center. PATIENTS: Patients treated for low rectal adenocarcinoma between January 1, 1994, and October 31, 1999. INTERVENTIONS: Anal manometric data were prospectively collected at the time of initial diagnosis and before ileostomy closure. MAIN OUTCOME MEASURES: Mean and maximum resting pressures (RPs) and squeeze pressures, threshold volume for sensation, and maximal tolerable volume. RESULTS: Twenty-three patients in the surgery group and 19 in the chemoradiotherapy group were considered for analysis; 15 patients had preoperative radiotherapy and 4 had postoperative radiotherapy. At the time of ileostomy closure, RPs were significantly lower in the chemoradiotherapy group than in the surgery group (32.7 +/- 17 vs 45.3 +/- 18 mm Hg; P =.03). Squeeze pressures were not significantly different between the surgery and chemoradiotherapy groups (108.7 +/- 56.7 vs 102.0 +/- 52.6 mm Hg; P =.69). The ratios of postresection to preresection RPs were also significantly lower in the chemoradiotherapy group (0.49 +/- 0.29) than in the surgery group (0.76 +/- 0.22) (P =.005). Eight to 12 weeks after proctectomy with coloanal anastomosis, a 24% decrease in RP was noted in the surgery group. The addition of adjuvant pelvic irradiation decreased RP by another 27%. CONCLUSION: Adequate shielding of the anal sphincter should be performed for low rectal cancers whenever a sphincter-preserving procedure is considered.