Primary care physicians and nurse practitioners are the initial casefinders of mental health problems and major providers of mental health treatment in the United States. However, past studies suggest that such primary care providers often neither recognize nor correctly diagnose their patients' mental disorders. This study compared an HMO's primary providers' direct assessments of the current emotional disorders of patients just seen for an outpatient medical visit with those of mental health professionals assessing the same patients with the Structured Clinical Interview for DSM III R (SCID). Using the SCID-derived diagnosis as the standard, the primary providers failed to recognize almost two-thirds of their patients with a current mental disorder. Although confident in their assessments, the primary providers were also able to correctly identify very few of the specific mental disorders most prevalent in primary medical care practice; they identified only one of the seven depressions, three of the 18 anxiety disorders, and none of the four alcohol or drug abuse disorders. Reasons for these diagnostic discrepancies, comparisons with past studies, and training to improve primary providers' diagnosis of mental disorders in their patients are discussed.