Background & Aim: Colorectal cancer is the third most common cancer and the third leading cause of cancer death. Most often it is derived from an adenomatous polyp. Therefore, detection and treatment of these baseline polyps could potentially decrease the prevalence and incidence of colorectal cancers. One of these screening procedures is colonoscopy, which is more Precise than others like. Barium enema in detecting primitive and small polyps. In addition, colonoscopy has the added benefit that lesion can be removed during the same procedure. Thus, this study was performed to determine the frequency of polyps and colorectal tumors among patients who underwent colonoscopy in Shariati hospital between November 2000 and November 2002. Patients and Methods: This retrospective analytical cross-sectional study was performed on 993 patients who underwent colonoscopy in Shariati hospital from November 2000 to November 2002. Method of sampling was by census. All patients were evaluated for demographic variables, location and size of polyps and tumors by using a checklist. The data were collected, described and analyzed using SPSS V.11.5 sofware. Chi² and Student t-test were performed in the data analysis. Results: Among 993 patients who were studied, 529 (53.3%) were males and 464 (46.7%) were females. The frequency of polyps and tumors in this study was 15.4% (153 of 993) and 4.8% (48 of 993), respectively. There was a significant association between the frequency of polyps and its location (P<0.01). The most common location of polyps was sigmoid (76.5%). Also, a significant association was observed between the frequency of tumors and its location (P<0.01). The most common location of tumors was again recto-sigmoid colon (70.8%). Additionally, polyps and tumors were respectively 1.6 and 1.7 times more prevalent in men than women. Conclusion: Regarding the most prevalent location of polyps and tumors, the use of fexible sigmoidoscopy may lead to detection and even treatment of many accessible polyps and tumors which could result in prevention of many colorectal cancers. Also colonoscopy is useful to detect lesions which were not detected by sigmoidoscopy and these inaccessible polyps and tumors can be bilopsied.