OBJECTIVES: To estimate the impact of Medicare Part D on prescription and medical utilization among Arizona’s senior dual eligible population. METHODS: This study was a retrospective analysis of changes in pharmaceutical utilization and physician visits among Arizona senior dual eligible (Medicaid and Medicare) beneficiaries between the ages of 66 and 80 as of January 1, 2006 relative to a comparison group (Medicaid beneficiaries between the ages of 50 and 62 as of January 1, 2006).  Medical and pharmacy claims from the Medicaid program from January 1, 2005 to December 31, 2007 were used in this analysis. Differences between groups with respect to over-the-counter (OTC) medications, benzodiazepines, total prescription utilization, generic medication utilization, and physician visits were estimated using generalized estimating equations. RESULTS: The dual eligibles and comparison group were similar in their level and trend of utilization of over-the-counter (OTC) medications and benzodiazepines in the pre-Part D period.  Following implementation of Part D, there was an immediate decline in utilization of both OTC medications and benzodiazepines in the dual eligibles relative to the comparison group (p<0.001).  An upward trend was observed for both groups during the pre-Part D period for total prescription utilization and generic medication utilization.   After the implementation of Medicare Part D, utilization of these drug classes was significantly lower among the dual eligibles relative to the comparison group (p<0.001).   Trends in physician office visits were similar for the entire study period.  During the first month of Part D, however, the dual eligibles had a significantly larger increase in physician visits over the previous month relative to the comparison group (p=0.001).  CONCLUSIONS: This study suggests that medication use for dual eligible Medicare beneficiaries was disrupted by the transition of outpatient drug benefits from Medicaid to Medicare Part D.