A valid and reliable vignette-based measure of DSM-III psychiatric diagnostic knowledge was administered to practicing primary care physicians (PCPs; generally, internal and family practice medicine) and mental health professionals (MHPs, in psychiatry and psychology). Recognition, diagnosis, and treatment recommendations were measured for 14 different disorders. Contrary to other reports, PCPs consistently recognized the presence of mental disorder and did so virtually as well as MHPs, although both PCPs and MHPs showed more under-recognition than over-recognition. Diagnostic accuracy, however, was substantially lower, with that of MHPs exceeding PCPs for the general classes of affective, anxiety, somatic, and personality disorders, but not for the organic disorders. In making specific diagnoses, significantly fewer PCPs than MHPs gave an accurate diagnosis for eight of the 14 disorders: dysthymic disorder, major depression with psychotic features, agoraphobia with panic attacks (marginally), generalized anxiety disorder, adjustment disorder with anxious mood (marginally), psychologic factors affecting physical condition, and two personality disorders. Overall, PCPs were most accurate in identifying organic disorders (81% correct), least accurate in identifying the personality disorders (14%), and intermediate in identifying the affective (47%), anxiety (49%), and somatic disorders (49%). In most cases, both PCPs and MHPs preferred referral to treatment in primary care, but more PCPs than MHPs recommended treatment in primary care for certain anxiety and somatic disorders. Some differences in the recommended use of antidepressants in primary care were also found. Implications for the provision of mental health care by primary care physicians are discussed.