PURPOSE: The removal of smooth muscle during stapled hemorrhoidopexy raises concerns regarding its effects on postoperative anorectal function. The purpose of this study was to evaluate the correlation between the amount of muscle removed and changes in anorectal manometry following stapled hemorrhoidopexy. METHODS: Patients with symptomatic II, III, or IV degree hemorrhoids that underwent stapled hemorrhoidopexy between January 2008 and May 2011 were included in this study. Anorectal manometry was performed preoperatively and at three months postoperatively. The resected doughnuts were examined histologically, and the thicknesses of muscle fibers were evaluated. RESULTS: Eighty-five patients (34 males) with a median age of 47 years were included. Muscularis propria fibers were identified in 63 of 85 pathologic specimens (74.1%). The median thickness of the muscle fibers was 1.58 +/- 1.21 mm (0 to 4.5 mm). The mean resting pressure decreased by approximately 7 mmHg after operation in the 85 patients (P = 0.019). In patients with muscle incorporation, there was a significant difference in mean resting pressure (P = 0.041). In the analysis of the correlation of the difference in anorectal manometry results ([the result of postsurgical anorectal manometry] - [the result of presurgical anorectal manometry]) to the thickness of muscle fibers, no significant differences were seen. No patients presented with fecal incontinence. CONCLUSION: Although the incidence of fecal incontinence is very low, muscle incorporation in the resected doughnuts following stapled hemorrhoidopexy may affect anorectal pressure. Therefore, surgeons should endeavor to minimize internal sphincter injury during stapled hemorrhoidopexy.