Background: Irritable bowel syndrome is a symptom-based condition defined by the presence of abdominal pain or discomfort, with altered bowel habits, the diagnosis could be reached using symptom-based diagnostic criteria without the need for exhaustive investigation; however interest in the potential for a missed organic GI disease in patients diagnosed IBS has increased over the last years. Objective: This study aimed to evaluate the role of colonoscopy in diagnosing organic diseases in patients with colonic symptoms compatible with Rome IV criteria of Irritable bowel syndrome as well as studying the effect of the presence or absence of alarm features on detecting organic lesions Patients and methods:  A cross sectional study was carried out on 50 patients fulfilling the Rome IV criteria for IBS. The study was conducted in in the department of Hepatogastroenterology, Al-Azhar university Hospitals from December 2018 to May 2019.      Patients were selected and divided into two equal groups: Group A:  fulfilled Rome IV criteria for IBS without alarm features, and Group B: included 25 patients fulfilled Rome IV criteria for IBS with alarm features. All patients underwent full colonoscopy along with biopsy from colon was taken for histopathological examination. Results: The mean age of participants was 40.12 years. Males were 27 (54%) while females were 23 (46%), Eleven of 50 patients (22%) fulfilling Rome IV criteria of IBS had organic colonic diseases. There was higher prevalence of organic GI lesions in patients who reported alarm features.  Microscopic colitis was the predominant organic disease among studied patients. IBS-D has the highest contribution to the total number of organic diseases. Conclusion: Up to one in six patients meeting criteria for IBS without alarm features in routine clinical practice may have underlying organic GI lesions, and this increased to one in four patients if alarm features were present .