The Medicare Prescription Drug, Improvement and Modernization Act (MMA) established a new Medicare “Part D” that gives people access to a private Medicare prescription drug plan. The new law has particular relevance for the 6.4 million low-income Medicare beneficiaries also enrolled in Medicaid. These beneficiaries – often referred to as “dual eligibles” had their prescription coverage shifted from Medicaid to private Part D plans starting January 1, 2006. As a result, many dual eligibles face increases in their out-of-pocket costs under Part D since Medicaid co-payment requirements in many states fall below the levels that most face under Part D. In light of these changes, the relative lack of data on the impact of copayment requirements in this population is surprising given that these patients are much poorer and sicker than the privately insured population and hence may be more vulnerable to the intended and unintended effects of implementing even nominal cost sharing of a few dollars. This pilot study aims to examine the impact of copayment levels on antidepressant use and adherence in dual-eligibles with major depression. This study will take advantage of the variation in copayment requirements (i.e. $0.50 to $3) across the 50 states that existed in the pre-Part D period to inform the research question. Using the 2003 Medicaid Analytic Extract files merged with Medicare claims we will examine whether dual eligibles in states with higher copayments will have lower antidepressant use and adherence than those in states with lower or no copayments. Our long-term goal is to examine the impact of Part D on medication access, health outcomes, and expenditures among dual eligibles.