OBJECTIVE Any alarm symptoms in patients with irritable bowel syndrome (IBS) should be carefully evaluated. Colonoscopy is a standard diagnostic procedure for evaluating the colonic mucosa and ruling out probable diseases responsible for patient symptoms. We analyzed the colonoscopy findings in patients with and without IBS. METHODS Ninety-six patients with IBS and 101 without IBS were consecutively enrolled in the study. All the patients in the IBS group met the Rome IV criteria, and underwent colonoscopy due to the appearance of red flags. The colonoscopy findings were compared between the 2 groups of patients. RESULTS The main indications for colonoscopy in the IBS group were progressive abdominal pain (36.7%), rectal bleeding with fresh blood (17.7%), and occult blood in stool (12.5%). In the non-IBS group, the most prevalent indicators were rectal bleeding with fresh blood (37.6%), colorectal cancer surveillance (21.8%), and abdominal pain (13.9%). The most common macroscopic findings in the 2 groups were hemorrhoids, polyps, and anal fissure. There were no statistically significant differences with respect to the microscopic and macroscopic findings between groups. CONCLUSIONS We concluded that the prevalence of organic lesions in the colon of patients with IBS was the same as that in the patients without IBS. The Rome IV criteria accurately predicted IBS. Additional evaluation through colonoscopy in IBS should be based on the presence of alarm features.