OBJECTIVES: To evaluate medication adherence of select lipid lowering and oral anti-diabetic medications in diabetic patients during the Medicare Part D benefit coverage gap. METHODS: This was a retrospective evaluation of pharmacy claims database. The evaluation consisted of standard Medicare Part D benefit patients at an 185,000 member managed care organization during January 1, 2006 through November 30, 2006. Study patients included anyone who encountered claims for both a 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor (statin) and an oral (PO) anti-diabetic medication during January 2006, and reached the coverage gap by November 30, 2006. Patients were excluded if they subsequently experienced catastrophic coverage or did not reach the coverage gap during 2006. Medication adherence was measured using pharmacy claims data to calculate medication possession ratios (MPR). A descriptive comparison of MPR during “pre-coverage gap days” and “coverage gap days” was conducted for the study population. A MPR of = 0.8 was considered adherent. RESULTS: Of approximately 13,000 standard Medicare Part D patients, 189 had claims for both a statin and a PO anti-diabetic during January 2006, and subsequently experienced the coverage gap. The number of patients reaching the “doughnut hole” or coverage gap in a given month ranged from 4 to 41, starting in April. The average “pre-gap” and “gap” periods were 242 days and 123 days, respectively. The average MPR for PO anti-diabetics during the “pre-gap” period was 24% higher than during the “gap” (0.92 vs. 0.70). For the statins, there was an 18% decrease in the average MPR between the “pre-gap” period and the “gap” period (0.87 vs. 0.72). CONCLUSION: The overall medication compliance (i.e. MPR) of statins and oral anti-diabetics decreased during the 2006 Medicare Part D benefit coverage gap period. On average, patients became more non-adherent with their statin and oral anti-diabetic therapies during the “gap” period.