There is accumulating evidence, both quantitative and qualitative, that pelvic irradiation adversely affects anorectal function. However, histologic evidence of sphincter injury has not been demonstrated. This study was designed to perform histologic assessment of collagen deposition and nerve alteration in the internal anal sphincters of rectal cancer patients who underwent abdominoperineal resection after adjuvant chemoradiation therapy and to correlate the degree of histologic changes with the time interval between chemoradiotherapy and abdominoperineal resection.