To assess the validity and utility of PRIME-MD (Primary Care Evaluation of Mental Disorders), a new rapid procedure for diagnosing mental disorders by primary care physicians. Survey; criterion standard. Four primary care clinics. A total of 1000 adult patients (369 selected by convenience and 631 selected by site-specific methods to avoid sampling bias) assessed by 31 primary care physicians. PRIME-MD diagnoses, independent diagnoses made by mental health professionals, functional status measures (Short-Form General Health Survey), disability days, health care utilization, and treatment/referral decisions. Twenty-six percent of the patients had a PRIME-MD diagnosis that met full criteria for a specific disorder according to the Diagnostic and Statistical Manual of Mental Disorders, Revised Third Edition. The average time required of the primary care physician to complete the PRIME-MD evaluation was 8.4 minutes. There was good agreement between PRIME-MD diagnoses and those of independent mental health professionals (for the diagnosis of any PRIME-MD disorder, kappa = 0.71; overall accuracy rate = 88%). Patients with PRIME-MD diagnoses had lower functioning, more disability days, and higher rates of health care utilization than did patients without PRIME-MD diagnoses (for all measures, P < .005). Nearly half (48%) of 287 patients with a PRIME-MD diagnosis who were somewhat or fairly well-known to their physicians had not been recognized to have that diagnosis before the PRIME-MD evaluation. A new treatment or referral was initiated for 62% of the 125 patients with a PRIME-MD diagnosis who were not already being treated. PRIME-MD appears to be a useful tool for identifying mental disorders in primary care practice and research.