The longitudinal comparison study of the national evaluation of the Comprehensive Community Mental Health Services for Children and Their Families Program assesses factors in service delivery systems that may shape the impact of systems of care. We evaluated the extent to which services embodying the principles of a system of care, as measured by the System-of-Care Practice Review (SOCPR), affect clinical outcomes for children being served in federally-funded systems of care and matched comparison communities. The participants included 75 children and families selected for participation in the SOCPR. Results indicated that experience of services embodying system-of-care principles at a high level was associated with lower symptom and impairment scores one year after entry into services. Children in systems of care were rated to have experienced high levels of system-of-care principles implemented within their services, and their symptom severity and functional impairment did not vary as a function of their experiences. Children in matched comparison communities had more variable rated experiences of system-of-care principles within their services, and their symptom severity and functional impairment decreased as their experiences of the principles increased.