BACKGROUND: For clinical or research use in primary care, the DSM-IV diagnostic criteria for somatization disorder are too restrictive, while the criteria for undifferentiated somatoform disorder are overly inclusive. In this article, we examine the validity of multisomatoform disorder, defined as 3 or more medically unexplained, currently bothersome physical symptoms plus a long (> or = 2 years) history of somatization. METHODS: Data from the Primary Care Evaluation of Mental Disorders Study of 1000 patients from 4 primary care sites were analyzed. The outcomes assessed were 6 domains of health-related quality of life, using the 20-item Short-Form General Health Survey; self-reported disability days and health care use; satisfaction with care; and physician-rated difficulty of the encounter. RESULTS: Multisomatoform disorder was diagnosed in 82 (8.2%) of the 1000 patients who were enrolled in the Primary Care Evaluation of Mental Disorders Study. Compared with mood and anxiety disorders, multisomatoform disorder was associated with comparable impairment in health-related quality of life, more self-reported disability days and clinic visits, and greater clinician-perceived patient difficulty. CONCLUSIONS: Multisomatoform disorder may be a valid diagnosis and potentially more useful than the DSM-IV diagnosis of undifferentiated somatoform disorder. Also, because multisomatoform disorder has a large and independent effect on impairment, its diagnosis should not be precluded simply because of a coexisting mood or anxiety disorder.