Using nationally representative data from 1995 and 2000, this study examines trends in managed care penetration and activity among outpatient drug treatment organizations in the United States. Further, it investigates how managed care activity varies across different types of treatment providers and for public and private managed care programs. Overall, managed care activity has increased, with a greater proportion of units having managed care arrangements and a larger percentage of clients covered by managed care. In general, public managed care activity has increased and private managed care activity has decreased. Treatment providers report that they have fewer managed care arrangements, which may reflect consolidation in the managed behavioral care sector. Finally, growth in managed care among outpatient substance abuse treatment units affiliated with hospitals and mental health centers may signal a preference for providers that can effectively link substance abuse treatment with medical and social service provision, or alternatively, that linkages with such organizations may provide the size necessary to assume the risks associated with managed care contracts.