Implementation of the Clinton Administration's proposed Health Security Act could significantly increase the number of seriously mentally ill persons enrolled in health maintenance organizations (HMOs). Studies of the mental health status and outcomes of seriously mentally ill persons treated in HMOs show conflicting findings and provkk little guidance about the likely effect of the Health Security Act on this population. However implementation of the Health Security Act would be likely to influence the treatment of seriously mentally illpersons by altering the activities of state and local governments in the financing and delivery of care, facilitating the integration of public and private services, and encouraging development of additional service capacity. To clarify HMOs' likely role inproviding services to seriously mentally ill persons under health care reform, more research is needed on the currrent service capacity of HMOs, on treatment approaches and outcomes in different HMO models, on whether seriously mentally ill persons would be likely to choose HMOs from among health plan options, and on whether higher-in-come HMO enrollees would seek care outside HMO provider networks.