This paper presents national estimates of behavioral disorders among Medicaid recipients. The 12-month prevalence of 14 disorders in the National Comorbidity Survey was 48% (43% mental, 14% substance use), and of 6 disorders in the National Household Survey on Drug Abuse was 27% (21% mental, 9% substance use). Total and specific disorder rates are from 50% to more than 100% higher in Medicaid than in the total population, and exceed most other insurance status groups. By promoting detection and treatment of these disorders in Medicaid, mental health administrators and policymakers may reduce barriers to education, employment, family stability, and departure from welfare.