OBJECTIVES: The objective was to determine the impact of Medicare part D doughnut hole or the ‘coverage gap’ on medication compliance behavior and healthcare resource utilization among Medicare beneficiaries taking statin medications. METHODS: This was a retrospective cohort study using a 2007 5% National sample of Medicare beneficiaries. Beneficiaries enrolled in the Part D program with at least 1 prescription claim for statin were included in the study. Patients were classified as continuers, switchers and discontinuers of statin medications after reaching the coverage gap. The primary outcomes of interest were average number of emergency room (ER) visits, hospitalizations, out of pocket (OoP) costs and number of prescriptions pre and post coverage gap. RESULTS: 131237 patients met the inclusion criteria. All patients in the study sample were diagnosed with hyperlipidemia, and 8,251 patients were prescribed with statins as a primary prevention treatment.   Of the beneficiaries taking statins, 65.8% were female; 80.71% white; 10.91% black and the mean age was 71.9 (±12.0) years. Overall, 24.3% of statin users reached the coverage gap in 2007, with 10.2% reaching the gap by March, 42.0% by June, and 74.4% by September. Within 90 days after reaching the gap, 74.2% of statin users continued using their statin medication, 1.0% switched to a different statin medication, and 24.8% discontinued use of statins. After reaching the coverage gap, the average number of ER visits/month increased from 0.24 to 0.25 and the average OoP costs/month increased from $58.51 to $104.03. CONCLUSIONS: The results suggest an overall negative impact of the Medicare part D coverage gap on enrollee health, adherence to statin medications, number of hospitalizations, emergency department visits and out of pocket expenses incurred by beneficiaries.  The disruption of the coverage gap on the quality of care is demonstrated by the large percentage of statin users discontinuing therapy.