The Primary Care Evaluation of Mental Disorders (PRIME-MD) was developed as a screening instrument but its administration time has limited its clinical usefulness. To determine if the self-administered PRIME-MD Patient Health Questionnaire (PHQ) has validity and utility for diagnosing mental disorders in primary care comparable to the original clinician-administered PRIME-MD. Criterion standard study undertaken between May 1997 and November 1998. Eight primary care clinics in the United States. Of a total of 3000 adult patients (selected by site-specific methods to avoid sampling bias) assessed by 62 primary care physicians (21 general internal medicine, 41 family practice), 585 patients had an interview with a mental health professional within 48 hours of completing the PHQ. Patient Health Questionnaire diagnoses compared with independent diagnoses made by mental health professionals; functional status measures; disability days; health care use; and treatment/referral decisions. A total of 825 (28%) of the 3000 individuals and 170 (29%) of the 585 had a PHQ diagnosis. There was good agreement between PHQ diagnoses and those of independent mental health professionals (for the diagnosis of any 1 or more PHQ disorder, kappa = 0.65; overall accuracy, 85%; sensitivity, 75%; specificity, 90%), similar to the original PRIME-MD. Patients with PHQ diagnoses had more functional impairment, disability days, and health care use than did patients without PHQ diagnoses (for all group main effects, P<.001). The average time required of the physician to review the PHQ was far less than to administer the original PRIME-MD (<3 minutes for 85% vs 16% of the cases). Although 80% of the physicians reported that routine use of the PHQ would be useful, new management actions were initiated or planned for only 117 (32%) of the 363 patients with 1 or more PHQ diagnoses not previously recognized. Our study suggests that the PHQ has diagnostic validity comparable to the original clinician-administered PRIME-MD, and is more efficient to use.