OBJECTIVES: Many retailers offer low cost generic prescriptions, which may mean Medicare patients pay cash for at least some Rxs. Plans typically have visibility only to adjudicated prescriptions and do not include cash prescriptions in their adherence metric.   This research investigates whether adherence is actually higher than that reported in the Medicare Advantage Star Ratings.  METHODS: Adheris® receives a nationally representative sample of prescription data, collected directly from retail pharmacies, containing roughly 40% of all U.S. retail prescription volume and 130 million unique patient IDs.  The HIPAA-compliant, longitudinal data captures all prescriptions filled at the pharmacy, including all payment methods. The study cohort selected all patients filling at least one non-cash prescription in the oral diabetes category between January and September 2013.  Patients were observed through December 2013.  Exclusion criteria used by CMS for the 2013 Part D oral diabetes medication adherence metric were employed.  Patient eligibility (requiring pharmacy use in the last quarter of the observation period) and a constant store panel were utilized to ensure completeness of patient data.  Medicaid patients were excluded. Adherence was calculated using proportion of days covered (PDC).  A patient is considered adherent if their PDC>=80%.  PDC computed using only non-cash prescriptions is compared to PDC computed using all prescriptions (including cash) to determine the impact of cash prescriptions on average PDC and on the percentage of patients considered adherent. RESULTS: In patients 65+ with at least 1 cash prescription (n=62,454), including cash prescriptions improves average PDC by 14.89% (64.09-78.98%) and the percentage of adherent patients by 19.38% (40.92-60.30%). In all patients 65+ (n=483,740), the improvement is 1.92% (76.45-78.38%) and 2.50% (58.16-60.66%) respectively.  CONCLUSIONS: Including cash prescriptions improves adherence rates reported by plans as part of the Star Ratings, indicating that patients are more adherent than other statistics indicate.