The Centers for Medicare & Medicaid Services (CMS) Medicare Health Plan Quality and Performance Ratings program, or Star Ratings program, includes 3 medication adherence outcome measures (Medication Adherence for Diabetes Medications, Medication Adherence for Hypertension, and Medication Adherence for Cholesterol), which contribute to approximately 31% of a Medicare Advantage prescription drug (MA-PD) plan's Part D summary rating and 12% of its overall star rating (a weighted summary of a plan's Part C and Part D ratings). MA-PD plans have been increasing their adherence intervention efforts as an approach to improving beneficiaries' medication adherence and the plan's quality performance ratings. However, few evaluations have examined the effects of these interventions on medication adherence and performance ratings.To assess the impact of a quasi-experimental multichannel adherence intervention on beneficiaries' medication adherence and health plan quality performance measures for 2 MA-PD plans. The intervention included a daily prescriber-directed 90-day retail refill component and a weekly member-directed refill reminder component.Members filling 1 or more medications assessed by the 3 CMS star ratings adherence measures (oral antidiabetics [DM], antihypertensives [HTN], and statins [CHOL]) were identified for a 2-component intervention starting in April 2013. The retail-based 90-day refill component sent populated 90-day prescription letters via fax to prescribers of members who had filled a 30-day prescription. One fax per member-medication instance was sent and included the option to fax approval of a new 90-day prescription. For prescribers who approved, members were notified that a 90-day refill was available at their retail pharmacies. The refill reminder component used weekly scans of pharmacy claims to identify members' refill patterns. Reminder letters were sent to members 7 days late to refill. An intent-to-treat approach was used to assess intervention effectiveness. Pharmacy claims and health plan enrollment data were used to calculate adherence among members enrolled 1 year pre- and postintervention start (April 2013) for the intervention group and a comparator group that consisted of 4 MA-PD plans not implementing an adherence intervention. Adherence was estimated by the proportion of days covered (PDC) and generalized linear models were fit to calculate difference-in-difference (DID) estimators to control for demographics, comorbidity, and changes in adherence over the study period. To estimate the impact of the intervention on health plan quality performance, 2013 and 2014 CMS star ratings for the respective 2012 and 2013 measurement years were compared by group.During the first year of the intervention, 1,344 prescribers representing 6,701 members were faxed 15,284 populated prescription letters. Prescriber response rate was 53.7% and approval rate (approved letters of total sent) was 47.3%. An average of 539 refill reminder letters were sent per week. DID estimators showed positive influence of the intervention on adherence to HTN and CHOL. For the intervention group, adherence increased 2.0 percentage points (P < 0.001) for HTN and 1.8 percentage points (P < 0.001) for CHOL, relative to the control group. The odds of achieving adherence (PDC ≥ 80%) were greater in the intervention group compared with control for HTN (ratio of odds ratio [ROR] = 1.334; 95% CI = 1.203-1.479) and for CHOL (ROR = 1.247; 95% CI = 1.132-1.374). For the 2 plans implementing the intervention, CMS-published adherence rates for DM, HTN, and CHOL increased from 2012 to 2013 by an average 5.5, 6.0, and 7.0 percentage points, respectively, compared with the control group (3.5, 2.0, and 1.8 percentage points, respectively).This study found increases in adherence to HTN and CHOL in 2 MA-PD plans implementing a combined prescriber and patient-directed intervention. MA-PD plans can use broad multichannel interventions to address common adherence barriers and as an approach to improving members' adherence to maintenance medications and CMS quality performance ratings.No funding was received in return for conducting this research. Leslie is an employee of MedImpact Healthcare Systems. The authors report no conflicts of interest. Study concept was developed primarily by Leslie, along with Gilmer. Leslie took the lead in data collection, while data interpretation was performed by Leslie, Natarajan, and Gilmer. The manuscript was written primarily by Leslie, along with Gilmer and assisted by Hovell. Leslie, Gilmer, and Hovell revised the manuscript, with assistance from Natarajan.