Background: To determine whether IBS patients develop organic lesions compared to those, without IBS, and to determine type and frequency of these organic colonic lesions. Material/Methods: Retrospective review of medical records of 622 IBS patients, ages 19-91 years, over fifteen years that underwent colonoscopy for new gastrointestinal symptoms during the course of their illness. Records of 642 non-IBS patients, who had colonoscopy for gastrointestinal complaints, were reviewed retrospectively as a comparison group. We abstracted and analyzed data related to demographics, history, diagnosis of IBS, and type of colonic lesions reported in the colonoscopy reports. Results: Of the 622 patients diagnosed with IBS, the median duration of the IBS was 11 years (range = 1 to 62 years). Colonoscopy findings were normal in 301 patients (48.4%) in the IBS group and 301 patients (46.9%) in the non-IBS group. Among the IBS group, the common organic colonic lesions were hemorrhoids (21.1%) polyps (20.3%) and diverticuli (19%) and angiodysplasia (11.9%). Among the non-IBS group, the common organic colonic lesions were hemorrhoids (22.6%), polyps (22.4%), diverticuli (20.6%) and angiodysplasia (12.1%). There was no difference in the prevalence of organic colonic lesions among patients with or without IBS (p > 0.05). Adjusting for the demographic variables and the number of lesions, there were no differences between the groups (p > 0.05). Conclusions: IBS patients may also develop organic colonic lesions, thus colonoscopy, if indicated, should not be delayed in these patients because of the assumption that their symptoms are due to IBS alone.