OBJECTIVES: The primary research question was to identify trends in cost of the six protected medication classes in the Part D program based on plan type and low income status between calendar years 2007 and 2008. The second research question was to identify trends in gap phase and catastrophic phase entry.     METHODS: The primary data source was the prescription drug event data in the Chronic Condition Warehouse (CCW) for 2007 and 2008.  Data were inflation-adjusted.  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: Beneficiaries enrolled in PDPs had higher drug costs per person across all drug classes for both years compared to those in MA-PDs. LIS enrollees had higher drug costs per person for each year for the drug classes compared to non-LIS enrollees with the exception of antineoplastics.  These findings were in the context of number of prescriptions filled per enrollee increasing less than one percent between calendar years 2007 and 2008 for all drug classes. Odds ratio point estimates between the two years for gap phase entry and catastrophic phase entry indicate that PDP and LIS enrollees were more likely to reach the gap and catastrophic phases as compared to their counterparts (i.e., MA-PD and non-LIS enrollees) for both calendar years.     CONCLUSIONS: Findings suggest that drug costs per person increased from 2007 to 2008. In general, the increase was higher for MA-PD enrollees (compared to PDP enrollees) as well as for LIS enrollees (compared to non-LIS enrollees).  A higher likelihood of gap phase and catastrophic phase entry existed for PDP enrollees (compared to MA-PD enrollees) and for LIS enrollees (compared to non-LIS enrollees) for all classes.