Background: The aim of this study was to determine the patterns and determinants of service use in severely mentally ill persons drawn from the National Institute of Mental Health Epidemiological Catchment Area (ECA) program, a community-based epidemiologic survey. This information provides a baseline against which to track ongoing changes in the US mental health service system. Methods: Severe mental illness (SMI) was defined according to US Senate Appropriations Committee guidelines. Comparisons were made with persons who had a mental disorder that did not meet these criteria (non-SMI). Sociodemographic factors, and 1-year volume and intensity of mental or addictive services use were determined. Differences between those who used services and those who did not were examined using logistic regression. Results: Persons with SMI differed from persons with non-SMI in most sociodemographic characteristics. A higher proportion of persons with SMI used ambulatory services, but the mean number of visits per person did not differ from the non-SMI population. Persons with SMI comprised the bulk of hospital inpatients admitted during a 1-year period. Several significant sociodemographic determinants of service use were found, with different patterns for general medical and specialty service use, pointing out potential barriers to care. Conclusions: As health care reform measures continue to be debated, attention to the service needs of the severely mentally ill is of crucial importance. Pre-managed care (pre-1993) baseline service use benchmarks will be essential to assess the impact of managed care on access to care, particularly for the severely mentally ill. Periodic collection of epidemiologic data on prevalence and service use would thus greatly facilitate service planning and addressing barriers to receiving mental health services in this population.