Users of mental health and substance abuse (MH/SA) services were examined among nonelderly high-cost Medicaid enrollees in 10 states in 1995. Although MH/SA service users constitute 11% of all Medicaid enrollees, they make up nearly a third of high-cost enrollees. Adults account for two thirds of this high-cost MH/SA group, and most frequently qualify for Medicaid through disability-related eligibility categories. In contrast, a majority of children in the high-cost MH/SA group are eligible for Medicaid through child-related categories, rather than disability. In diagnostic makeup, the high-cost group was somewhat more likely to have serious disorders than the general Medicaid MH/SA user population.