Objective - To investigate the form, frequency, management and outcome of common psychological disorders in primary care patients. Design - Two-stage sampling of primary care attenders followed by 3 and 12 month longitudinal assessment of mental state disability and treatment carried out using the same methods in 15 centres around the world, and coordinated by the World Health Organization. Main measures - The General Health Questionnaire, the primary care version of the Composite International Diagnostic Interview used to derive ICD-10 diagnosis, the Groningen Social Disability Schedule, and the primary care physician's assessment of the patient's current physical and psychological status along with a summary of their management of the case. Results - 25,916 patients were screened and 5,438 patients were interviewed in detail. Mental disorders among general health care attenders were common (average 24% of consecutive attenders, range 7.3%-52.5%). Disability was increased in patients with psychological illness: the more severe the psychological illness, the more severe was the disability The main presenting problem of such patients was often somatic, and only a minority presented a clearly psychological symptom. Detection varied greatly between centres, and across all centres half of the ICD-10 cases were not detected by the attending physician. Doctors in Verona displayed a particular bias towards psychological disorder. Treatment was offered to almost all patients identified as having psychological disorder by the primary care physician, though treatments showed a similar distribution regardless of diagnosis. Conclusions - The frequency and disability associated with the psychiatric disorder in general health care emphasise their importance to primary care and public health. These are patients who present to primary care, and most continue to be managed in primary care without recourse to specialist mental health services. It is important that training about the detection, diagnosis and management of common mental disorders are strengthened at medical school and in the vocational training of general practitioners.