Aims This paper describes the methods used in a study of the prevalence and types of common mental disorders among patients attending New Zealand general practices, and reports some key findings from the first phase of the study. The study also aimed to determine the degree of associated disability and other factors influencing recognition, management, course and outcome of these disorders, and subsequent papers will address these issues. Methods General practitioners (GPs) were selected randomly. In the first phase of the study, all adult attenders at each practice on selected days were administered a short questionnaire, the General Health Questionnaire (GHQ-12), which screens for psychological symptoms. The GP recorded the reasons for each consultation, and was interviewed at the end of each day about selected patients to determine their opinion about the type of psychological problems experienced. Selected patients were then visited in their own homes and an extensive interview conducted, which included the Composite International Diagnostic Interview (CIDI) to determine mental health status, the World Health Organization’s Disability Assessment Schedule (WHODAS) to determine disability, and a detailed exploration of use of health services. In the second phase of the study, patients were contacted by telephone at three, six, nine and 12 months, and both patients and GPs were re-interviewed at 12 months. Results The study achieved a very high response rate among the GPs (90%). Nearly all eligible patients (93%) completed the GHQ screening, and their response rate was 70% for the first-phase interview. GPs thought that 54% of female and 46% of male patients had experienced some level of psychological problems in the past year. GHQ screening also found that more than half of those attending their general practitioner experienced some psychological symptoms at initial screening, and the CIDI interview found that more than one in three had a diagnosable mental disorder during the past 12 months. The most common mental disorders were depressive, anxiety and substance use disorders. These disorders were more common among younger than older general practice attenders, and comorbidity was high. Conclusions Mental health problems are very common among general practice attenders. Contrary to the prevailing view that general practitioners seldom identify psychological problems in their patients, they identified about half their patients as having some type of psychological problems in the past year, although they considered that these were moderate or severe in about only one in ten patients. Further work from this large New Zealand study will focus on the nature of the relationship between disorder and disability, and on the recognition, management and outcome of psychological problems.