BACKGROUND & AIMS: Irritable bowel syndrome (IBS) imposes significant clinical and economic burdens. We aimed to characterize practice patterns for patients with IBS in a large health maintenance organization, analyzing point of diagnosis, testing, comorbidities, and treatment. METHODS: Members of Kaiser Permanente Northern California who were diagnosed with IBS were matched to controls by age, sex, and period of enrollment. We compared rates of testing, comorbidities, and interventions. RESULTS: From 1995-2005, IBS was diagnosed in 141,295 patients (mean age, 46 years; standard deviation, 17 years; 74% female). Internists made 68% of diagnoses, gastroenterologists 13%, and others 19%. Lower endoscopy did not usually precede IBS diagnosis. Patients with IBS were more likely than controls to have blood, stool, endoscopic, and radiologic tests and to undergo abdominal or pelvic operations (odds ratios, 1.5-10.7; all P < .0001). Only 2.7% were tested for celiac disease, and only 1.8% were eventually diagnosed with inflammatory bowel disease. Chronic pain syndromes, anxiety, and depression were more common among IBS patients than among controls (odds ratios, 2.7-4.6; all P < .0001). Many patients with IBS were treated with anxiolytics (61%) and antidepressants (55%). Endoscopic and radiologic testing was most strongly associated with having IBS diagnosed by a gastroenterologist. Psychotropic medication use was most strongly associated with female sex. CONCLUSIONS: In a large, managed care cohort, most diagnoses of IBS were made by generalists, often without endoscopic evaluation. Patients with IBS had consistently higher rates of testing, chronic pain syndromes, psychiatric comorbidity, and operations than controls. Most patients with IBS were treated with psychiatric medications.