To examine factors associated with Medicaid acceptance for substance abuse (SA) services by outpatient SA treatment programs. Secondary analysis of 2003-2006 National Survey of Substance Abuse Treatment Services data combined with state Medicaid policy and usage measures and other publicly available data. We used cross-sectional analyses, including state fixed effects, to assess relationships between SA treatment program Medicaid acceptance and (1) program-level factors, (2) county-level sociodemographics and treatment program density, and (3) state-level population characteristics, SA treatment-related factors, and Medicaid policy and usage. State Medicaid policy data were compiled based on reviews of state Medicaid-related statutes/regulations and Medicaid plans. Other data were publicly available. Principal Findings. Medicaid acceptance was significantly higher for programs: (a) that were publicly funded and in states with Medicaid policy allowing SA treatment coverage; (b) with accreditation/licensure and nonprofit/government ownership, as well as mental- and general-health focused programs; and (c) in counties with lower household income. SA treatment program Medicaid acceptance related to program-, county, and state-level factors. The data suggest the importance of state policy and licensure/accreditation requirements in increasing SA program Medicaid access