The impact of criminal justice involvement and clinical characteristics on the cost of public treatment services for adults with serious mental illnesses is unknown. The authors examined differential effects of justice involvement on behavioral health treatment costs by primary psychiatric diagnosis (schizophrenia or bipolar disorder) and also by substance use diagnosis among 25,133 adult clients of Connecticut’s public behavioral health system in fiscal years 2006 and 2007. Justice-involved adults with schizophrenia had the highest costs, strongly driven by forensic hospitalizations. Addressing the cross-system burdens of forensic hospitalizations may be a sensible starting point in the effort to reduce costs in both the public behavioral health and justice systems.