The World Health Organization collaborative study on "Psychological Problems in General Health Care" investigated the form, frequency, course and outcome of common psychological problems in primary care settings at 15 international sites. The research employed a two-stage case-finding procedure. GHQ-12 was administered to 25916 adults who consulted health-care services. The second-stage assessment (n = 5438) consisted of the Composite international Diagnostic Interview (CIDI), the Social Disability Schedule, and questionnaires. Possible cases or borderline cases of mental disorder, and a sample of known cases, were followed up at three months and one year. Using standard diagnostic algorithms (ICD-10), prevalence rates were calculated for current disorder (one-month) and lifetime experience disorder. Well-defined psychological problems are frequent in all the general health-care settings examined (median 24.0%). Among the most common were depression anxiety, alcohol misuse, somatoform disorders, and neurasthenia. Nine per cent of patients suffered from a "subthreshold condition" that did not meet diagnostic criteria but had clinically significant symptoms and functional impairment. The most common co-occurrence was depression and anxiety. Comorbidity increases the likelihood of recognition of mental disorders in general health care, and the likelihood of receiving treatment.