Microscopic Colitis (MC) and diarrhea predominant irritable bowel syndrome (IBS-D) has almost similar clinical feature but MC is diagnosed by histologic criteria and IBS is diagnosed by symptom-based criteria. There is ongoing debate about the importance of biopsies from endoscopically normal colonic mucosa in the investigation of patients with IBS-D. Aim of this study was to assess the prevalence of MC in patient with IBS-D and to determine the distribution of MC in the colon. This observational study was conducted in department of Gastroenterology, Bangabandhu Sheikh Mujib Medical University (BSMMU), Dhaka, Bangladesh from January 2008 to December 2009. Patients were evaluated thoroughly & who meet Rome-II criteria with normal routine laboratory tests, were included in the study. Colonoscopy was done and biopsies were taken from the caecum, transverse colon, descending colon, and rectum. Out of total 60 patients, 22 had Lymphocytic Colitis (LC), 28 had nonspecific microscopic colitis (NSMC) and 10 had irritable bowel syndrome noninflamed (IBSNI). The distribution of LC was restricted to proximal colon in 15 patients, in the left colon in 2 patients and diffuses throughout the colon in 5 patients. There is considerable symptom overlap between the patients of IBS-D and patients with microscopic colitis. Without colonoscopic biopsy from multiple sites, possibility of MC cannot be excluded in patients with IBS-D and it can be said that clinical symptom based criteria for irritable bowel syndrome are not sufficient enough to rule out the diagnosis of microscopic colitis.