Irritable bowel syndrome is a very common clinical problem for which there are no established diagnostic criteria. The two aims of the present study were first, to create a scoring system for the diagnosis of irritable bowel syndrome incorporating features from the case history, physical examination, and some basic investigations including erythrocyte sedimentation rate and blood count; and second, to estimate the diagnostic accuracy of this scoring system in terms of its sensitivity and specificity, and of its predictive value in patients diagnosed as having irritable bowel syndrome by conventional methods. A group of 479 consecutive outpatients referred to a gastrointestinal clinic was studied by the usual extensive routine workup practiced at the clinic, including numerous laboratory tests, endoscopy of upper and lower gastrointestinal tract, and ultrasound. In 209 patients, symptoms were related to an underlying organic disease and in 108 patients irritable bowel syndrome was diagnosed without any recognizable organic disease. As a separate exercise, a scoring system including the answers of a questionnaire and the results of eight basic investigations was established. After completion of the study, the score was weighted by means of a logistic regression analysis. Using the weighted score, at a sensitivity of 64%, the specificity for the diagnosis of irritable bowel syndrome was 99%, and at a sensitivity of 83%, the specificity was 97%. Patients with organic diseases such as malignant tumor or inflammatory bowel disease were well discriminated by the score. It is concluded that a detailed history, physical examination, and basic laboratory tests are sufficient in most patients for the positive diagnosis of irritable bowel syndrome and the exclusion of any underlying organic diseases.