['Irritable bowel syndrome (IBS) is diagnosed by the presence of a constellation of symptoms fulfilling the Manning or Rome Criteria, after exclusion of organic disease. To exclude other diagnoses that might contribute to the abdominal pain or bowel symptoms experienced by subjects with IBS, numerous screening algorithms have been advocated, incorporating lactose hydrogen breath tests, thyroid function tests, fecal ova and parasite determination, and colonic endoscopy/radiography. The utility of these tests in uncovering alternative diagnoses, other than IBS, was examined in 1452 patients.', 'Data were combined from two large multinational studies of IBS patients. All patients exhibited symptoms meeting the Rome criteria for IBS for at least 6 months before study entry. If prior evaluation had been > 2 yr previously, patients underwent colonic endoscopy/radiography at study entry. In addition, thyroid function tests, fecal ova and parasite determination, and a lactose hydrogen breath test were performed.', 'Lactose malabsorption was diagnosed in 23% (256/1122) of patients. Colonic abnormalities were detected in 2% (7/306) of patients; in four patients, colonic inflammation (n = 3) or obstruction (n = 1) may have contributed to symptoms of abdominal pain or altered bowel habits. Abnormal thyroid-stimulating hormone levels were detected in 6% (67/1209) of patients, of whom half were hypothyroid and half were hyperthyroid. Positive fecal ova and parasite tests were noted in 2% (19/1154) of patients.'] Do additional investigations fail to alter the diagnosis of irritable bowel syndrome in subjects fulfilling the Rome criteria?