The National Institute of Mental Health multisite Epidemiologic Catchment Area (ECA) program is described in the context of four previous psychiatric epidemiologic surveys that included a combined total of 4,000 subjects from Stirling County, the Baltimore Morbidity Study, Midtown Manhattan, and the New Haven third-wave survey. The ECA program is distinguished by its sample size of at least 3,500 subjects per site (about 20,000 total); the focus on Diagnostic Interview Schedule--defined DSM-III mental disorders; the one-year reinterview-based longitudinal design to obtain incidence and service use data; the linkage of epidemiologic and health service use data; and the replication of design and method in multiple sites. Demographic characteristics of community and sample populations are provided for New Haven, Conn, Baltimore, and St Louis.