The European Study of the Epidemiology of Mental Disorders (ESEMeD/MHEDEA 2000) is a new cross-sectional study investigating the prevalence and the associated factors of mental disorders, as well as their effect on health-related quality of life and the use of services in six European countries. This paper describes the rationale, methods and the plan for the analysis of the project. A total of 22,000 individuals representative of the non-institutionalized population aged 18 and over from Belgium, France, Germany, Italy, the Netherlands and Spain are being interviewed in their homes. Trained interviewers use a computer-assisted personal interview (CAPI) including the most recent version of the Composite International Diagnostic Interview (CIDI, 2000), a well-established epidemiological survey for assessing mental disorders. This is the first international study using the standardized up-to-date methodology for epidemiological assessment. Sizeable differences in prevalence, impact and level of need that is met by the health services are expected. The analysis of these differences should facilitate the monitoring of ongoing mental health reform initiatives in Europe and provide new research hypotheses.