OBJECTIVES: Approximately 4.1% of the U.S. adult population suffers from serious mental illness (SMI). Those dually eligible for Medicare and Medicaid suffer disproportionately from SMI (~30%). Antipsychotic medications can significantly improve the lives of people with SMI, but can have a variety of adverse effects, which may be exacerbated by polypharmacy. Antipsychotic nonadherence is also problematic. The real-world observation of antipsychotic utilization patterns has been identified as a priority. We assessed utilization of antipsychotics and other psychotropic medications among dually-eligible adults with SMI, including calculations of adherence, discontinuation, switching and polypharmacy. METHODS: The Medicare Part D Drug Event (PDE) file was used to identify Washington State dual-eligibles who filled an antipsychotic prescription in 2010. Twelve-month antipsychotic utilization rates were broken down by class (typical or atypical), and by medication. Adherence was measured using the proportion-of-days-covered methodology. For nonadherent beneficiaries we distinguished between those who discontinued use and those who switched to another antipsychotic. Rates of antipsychotic polypharmacy in a given 30-day period, and antidepressant and mood-stabilizer utilization within the 12-month observation period were estimated. RESULTS: There were 21,749 WA State dual-eligibles who filled an antipsychotic prescription in 2010. Over the 12-month observation period, 92% of the sample filled an atypical antipsychotic prescription (adherent=61%, discontinued=9%, switched=30%), and 16% filled a typical antipsychotic prescription (adherent=50%, discontinued=11%, switched=39%). Of the 4,307 duals who used more than one antipsychotic within a 30-day window, 91% used a maximum of two medications and only 9% used three or more. Finally, during the 12-month observation period, 46% of duals filled an antidepressant prescription and 23% filled a mood-stabilizer prescription. CONCLUSIONS: This information is critical for policymakers looking to improve quality of care and reduce costs, given that individuals with SMI often avoid clinical trials and duals accumulate roughly $300 billion [2013 USD] in annual healthcare expenditures.