OBJECTIVE: Anorectal function is greatly disturbed after rectal surgery with or without radiotherapy (RT). To clarify the underlying mechanisms, we designed a prospective study to evaluate the effect of RT and surgery on anorectal function and clinical outcome of patients with a rectal carcinoma. METHODS: Thirty-four patients with a rectal carcinoma participated in this study. They filled out a symptom questionnaire and underwent anal manometry, anal and rectal mucosal electrosensitivity testing, and a rectal barostat, before surgery, 4 and 12 months postoperatively. Thirteen patients were lost to follow-up, 14 underwent surgery alone (total mesorectal excision [TME]), and seven also received RT (RT + TME). RESULTS: Functional outcome was disappointing in both groups, with at 4 months a significantly higher defecation frequency after RT + TME as compared with TME. Anal sphincter function and rectal sensitivity to pressure-controlled distention were not affected by either treatment. Rectal compliance, however, was significantly reduced after RT + TME at 4 and 12 months, resulting in lower rectal volumes at the thresholds for first sensation and desire to defecate. Rectal but not anal mucosal electrosensitivity was higher after TME + RT. CONCLUSIONS: Anorectal function after rectal surgery with or without RT is greatly hampered because of a decreased rectal compliance. After 12 months, partial improvement is shown, especially in the absence of RT. © 2002 by Am. Coll. of Gastroenterology.