OBJECTIVES: To assess trends in the utilization and cost of the six protected medication classes in the Part D program between calendar years 2006 and 2007. METHODS: The primary data sources were the prescription drug event data in the Chronic Condition Warehouse (CCW) matched to the Beneficiary Annual Summary for 2006 and 2007.  Data were inflation-adjusted for 2007 costs.  Results are based on analysis of 100% data for Medicare beneficiaries in the CCW.  The six protected medication classes under study were anticonvulsants, antidepressants, antineoplastics, antipsychotics, antiretrovirals, and immunosuppressants.  RESULTS: Immunosuppressants were utilized by the fewest number of beneficiaries for 2006 and 2007 (81,974 and 80,187, respectively); antidepressants were utilized the most (6,040,698 and 6,645,639, respectively).  Immunosuppressants had the lowest aggregate ingredient costs for both years ($204 million and $199 million, respectively).  Antipsychotics had the highest ($4,161 million and 4,807 million, respectively). Ingredient costs to total Medicare expenditures ranged from 4.2% for antidepressants to 25.2% for antiretrovirals in 2006.  Results were similar for 2007 (4.5% for antidepressants to 29.4% for antiretrovirals).  From 2006 to 2007, antidepressants had the lowest increase and antiretrovirals the highest. Regarding ingredient cost payment per beneficiary, antiretrovirals had the highest increase from 2006 to 2007 at 17.6% while immunosuppressants decreased by -0.2%. CONCLUSIONS: Between 2006 and 2007, beneficiary utilization based on number of users per protected class increased for all classes except for immunosuppressants.  Aggregate ingredient cost payments increased for all six protected classes.  Based on ingredient cost payments per beneficiary, the immunosuppressant class was the only protected class showing a slight decrease; all other classes showed an increase.