OBJECTIVES: To examine the relationship between demographic and socio-economic factors and medication adherence (MA). METHODS: This study used a large nationally representative administrative claims database supplemented with socio-economic and community resource data.  The sample consisted of Medicare Advantage members enrolled in 2013. Multivariate logistic regression was used to determine factors associated with 3 MA measures from the CMS Five-Star Quality Rating System: Cholesterol (Statins) (MA-C), Diabetes Medications (MA-D) and Hypertension (RAS Antagonists) (MA-H). RESULTS: A total of 764,581 members from 44 health plans qualified for at least 1 of the measures (MA-C=68.0%, MA-D=72.2% and MA-H=74.0%). For all 3 measures, MA was significantly lower for members who were: younger and had a disability (OR: 0.54-0.95); African-American (OR: 0.64-0.66) (and, for MA-C and MD-D only, Hispanic (OR: 0.79-0.90)).  MA was lower for those who resided in an area with higher percent of population below federal poverty level (OR: 0.77-0.91), and was significantly higher for members who resided in an area with higher home ownership (OR: 1.05-1.08) or higher education level (OR: 1.04-1.07). After controlling for socio-economic and clinical (measured by number of different medications the member was taking) characteristics, MA was significantly higher for dual eligible members, with full benefit duals being more adherent (OR: 1.09-1.16) than partial duals (OR: 1.07-1.09). In addition, sub-group analyses suggested that non-duals who were low income actually had lower adherence than duals who were poor (but have more benefits due to dual status). Primary care shortage area was not a significant risk factor for MA. CONCLUSIONS: MA is significantly associated with demographic and socio-economic factors. Health plans serving a high proportion of disadvantaged members may be providing better quality of care than their MA measure results suggest. Specifically, dual members had higher adherence rates than members with similar characteristics who did not receive Medicaid benefits.