Hemorrhoidectomy is the treatment of choice for patients with third or fourth-degree hemorrhoids. Although the majority of surgeons believe that surgical hemorrhoidectomy is the most effective approach with excellent results in the management of hemorrhoid disease, but hemorrhoidectomy is not a simple procedure. One of the complications of this surgery is an injury to anal sphincters that can lead to incontinency in some patients. In this study, we aimed to reveal the percentage of external and internal anal sphincter injuries in surgical hemorrhoidectomy. We prospectively enrolled 128 patients from April 2006 to February 2007. They underwent hemorrhoidectomy in three general hospitals in Tehran. All patients were in grade III or IV and underwent open hemorrhoidectomy (Milligan-Morgan). After surgery, all resected material was histopathologically examined by two expert pathologists and the results confirmed by other one if there is any discrepancy. From all specimens which sent to the pathology department 15.8% (21 Pts.) had muscle fibers that Smooth muscle fibers were seen in 80.5% (17 Pts.) of them and striated muscle fibers were found in 19.5% (4 Pts.). Although hemorrhoidectomy is a safe and effective method for treatment of hemorrhoid, but the inadvertent removal of smooth and striated muscle during open hemorrhoidectomy had raised concerns about its effects on postoperative anorectal function.