OBJECTIVES: To examine whether utilization of antidepressants by dual eligibles were different under Part D compared to Medicaid period by focusing on states which vary baseline state Medicaid policies and wrap-around programs for dual eligibles to access drugs under Part D. METHODS: A pre-post study design with a longitudinal dataset by linking Medicaid data for 2004-2005 and 5% random sample of Medicare data for 2006-2007. The study population is dual eligibles, existing users of antidepressants in 2004 and with enrollment from 2004-2007 in eight states. We employ a state-fixed effect model to estimate medication utilization using proportion of days covered (PDC), adjusting for beneficiaries characteristics and health status.  We adopt generalized estimation equation (GEE) model for estimation and spline regression for investigating whether changes in PDCs is related to Part D.  A stratified analysis is conducted for community based dual eligibles (n=4,703) and nursing home dual eligibles (n=1,504) separately. RESULTS: Adjusted PDCs increased to 4.25 percentage point among community dual eligibles (p=0.00) but did not change among nursing home dual eligibles (p=.74) under Part D. Adjusted changes in PDCs were not significant for most states, except Arkansas community based dual eligibles (p=0.00), Florida nursing home dual eligibles (p=0.03) and New Mexico nursing home dual eligibles (p=0.01), which had significant increases under Part D.  Separate spline regression for these states indicated that trends in PDC before and after Part D were not significantly different, suggesting that changes were not related to transition to Part D. CONCLUSIONS: We did not find empirical support for concerns regarding disruption of medication utilization of the dual eligible under Part D. Although states had different baseline Medicaid polices and wrap-around programs under Part D, lack of significant changes in utilization suggest that minor changes in copayments and refill/prescription limits etc. do not have large effect on medication utilization to antidepressants.