This article describes a large longitudinal multicenter collaborative study that investigated the form, frequency, course, and outcome of psychological problems that were seen in primary health care settings in 15 different sites around the world. The research employed a two-stage sampling design in which the 12-item General Health Questionnaire was administered to 26422 persons aged 18 to 65 years who were consulting health care services. Of these persons, 5604 were selected for detailed examinations using standardized instruments and were followed up at 3 months and 1 year to provide information on course and outcome. All assessment instruments have been translated into 13 different languages. The project has produced a database that allows for the exploration of the nature of psychological disorders experienced by patients in general medical care and their association with physical illness, illness behavior, and
disability over time.