This report describes the characteristics of 123,240 individuals booked into jail in 2013 who were recent or former clients of the Washington State Department of Social and Health Services (DSHS) or the Health Care Authority (HCA). We compare mental health and substance use disorder treatment needs among the 44,805 Medicaid enrollees booked into jail and the general Medicaid population. To identify behavioral health treatment needs among Medicaid recipients booked into jail, we matched statewide jail booking records with mental health and substance use disorder indicators in the DSHS Integrated Client Database (ICDB). Mental health treatment needs were identified by receipt of publicly funded mental health treatment services, prescriptions for psychotropic medications, and mental health-related medical service encounters, hospitalizations and diagnoses during the jail booking year or the year prior. Substance use disorder treatment needs were identified using a comprehensive set of indicators including diagnoses, procedures, prescriptions, treatment or arrests that reflect a possible substance use disorder.