Background: Medicare Part D, which provides prescription drug coverage to Medicare beneficiaries, is delivered through either Medicare Advantage prescription drug (MA-PD) plans or stand-alone prescription drug plans (PDPs). MA-PD plans cover both drug therapy and other medical services, whereas PDPs provide prescription drug coverage only. Because of the potential substitutability between prescription drugs and other medical services, MA-PD plans may make greater efforts to improve enrollees' adherence to recommended medications than PDPs. Prescription drug benefits are more generous in MA-PD plans than in PDPs. OBJECTIVE: To assess statin adherence, comparing Medicare beneficiaries in MA-PD plans with those in PDPs. Methods: We used records from the Chronic Condition Warehouse 2007 Prescription Drug Event (PDE) file, associated Plan Characteristics files, and the Beneficiary Summary File (BSF) for a 5% random sample of Medicare beneficiaries. The study sample comprised Medicare beneficiaries aged 65 years or older in 2006 who filled at least 1 prescription for a statin during 2007, excluding beneficiaries with low-income subsidy or end-stage renal disease and those without both Medicare Part A and Part B enrollment in 2007. Medication adherence was measured by medication possession ratio (MPR), defined as the sum of days supply for all statin prescriptions filled in 2007 minus the days supply that would have carried over into 2008 from the final 2007 prescription filled, divided by the total number of days from the fill date of the first statin prescription to December 31, 2007. A binary indicator of good adherence was defined as MPR exceeding 80%. Propensity-score matching was used to reduce differences in observed characteristics of enrollees in MA-PD plans and PDPs. The propensity score was based on sociodemographic characteristics and health risk measures, including Hierarchical Condition Category (HCC) scores. Results: In the unmatched sample, the mean MPR was 70.57% for MA-PD enrollees versus 70.54% for PDP enrollees (P = 0.780), and the proportion of enrollees with good adherence was 46.7% for MA-PD plans versus 46.9% for PDPs (P = 0.262). In the matched sample, statin adherence was slightly better among MA-PD enrollees than PDP enrollees. Mean MPRs were 70.80% and 69.44%, and the percentages of enrollees with good adherence were 47.0% and 45.3% in MA-PD plans and PDPs, respectively (both P < 0.001). Conclusions: During an early year of the Part D program, MA-PD enrollees had slightly better adherence to statin therapy than PDP enrollees. While the difference was statistically significant, it was very small and unlikely to lead to clinically meaningful consequences. Less than one-half of MA-PD and PDP enrollees had good adherence in statin use, suggesting room for improvement in both types of Part D plans. Continuing evaluations of adherence in diverse therapy classes are needed for Medicare Part D beneficiaries.