The aims of the present study were: (1) to assess the prevalence of symptom-based irritable bowel syndrome (IBS) in Korean adults, (2) to assess several organic abnormalities which can be found in IBS patients, and (3) to analyze the risk or associated factors that influence the presence of IBS. Adult health examinees were requested to fill out a questionnaire. The prevalence of IBS was calculated using Rome II criteria. Using several tests, several organic abnormalities were identified in the IBS group. Risk factors were analyzed by comparing the IBS and normal groups. The prevalence value for IBS according to Rome II criteria was 16.8%. Mucosal hyperplasia, lymphocyte aggregation, and increased eosinophil counts were relatively common microscopic findings in IBS group. Female gender, self-consciousness of IBS, and irregular defecation were expressed as significantly independent risk or associated factors for IBS. Several colonic microscopic findings mentioned above may be helpful in accurate diagnosis of IBS. Therefore a more-precise and large population study about these findings is necessary to reach a definitive conclusion.