OBJECTIVES: This study assessed the association of the Medicare Part D coverage gap with medication adherence among beneficiaries with chronic obstructive pulmonary disease (COPD). STUDY DESIGN: Retrospective observational study based on Medicare claims data. METHODS: A 5% random sample of Medicare claims data (2006-2010) was used in this study. Beneficiaries diagnosed with COPD and treated with long- Acting bronchodilators (LABDs) were assigned to an exposure cohort (at risk of the coverage gap) or a control cohort (otherwise). The exposure and control cohorts were matched using high-dimensional propensity scores. Adherence was defined as ≥80% of the proportion of days covered by LABDs. Logistic regressions controlling for unbalanced covariates post matching were applied to assess the association of the coverage gap with adherence. RESULTS: The final matched exposure and control cohorts each included 4147 patient-year observations with about 42% and 46% of them adherent to LABDs, respectively. About 17% of the exposure cohort hit the coverage gap after October 31. Logistic regression showed that, compared with the control cohort, the beneficiaries in the exposure cohort had a significantly lower likelihood of being adherent if they hit the coverage gap later in the year (odds ratio [OR], 0.603; 95% CI, 0.493-0.738), or had a lower likelihood without statistical significance if otherwise (OR, 0.931; 95% CI, 0.846-1.024). CONCLUSIONS: The findings suggest that the Part D coverage gap was associated with lower adherence in patients with COPD, which may serve as evidentiary support for phasing out the coverage gap by 2020.