Health plans and providers can increase quality by improving adherence to chronic disease medications included in star ratings among Medicare Advantage Part D (MAPD) plan enrollees. Research is needed to evaluate effective means of collaboration between health plans and providers. The Medication Adherence Tracker (MAT) is a health plan initiative to help primary care providers use outreach to improve their patients' adherence. To quantify the contribution of structural and process factors on the success of a health plan-initiated tracking system in improving chronic disease medication adherence over 6 months. The MAT quality improvement initiative was carried out in South Texas from June to December 2016. Health plan pharmacists used claims data to identify MAPD enrollees at risk of nonadherence to triple-weighted star medications: renin-angiotensin system antagonists, oral diabetes medications, and statins. Actionable reports were delivered biweekly to each provider, either by fax or in person, by embedded health plan nurses. Multivariable regression was used to evaluate sociodemographic and clinical factors as well as the role of provider outreach in increasing paid pharmacy claims and medication adherence as measured by proportion of days covered (PDC) > 0.8. Of 3,542 patients in 5 Texas physician-organized delivery system groups whose 67 providers received tracking reports from June through December 2016, 1,901 (54%) patients had more than 1 related prescription, and 3,064 (87%) received provider outreach on at least 1 prescription. 2,493 (70%) had at least 1 paid pharmacy claim. Provider outreach was associated with greater likelihood of paid prescription claims (relative risk [RR] = 4.59, 95% CI = 3.74-5.62) and greater year-end adherence (PDC > 0.8, RR = 1.86, 95% CI = 1.63-2.12) in multivariable predictive models. 95% CIs for age, gender, low-income subsidy eligibility, and number of prescriptions did not exclude the null value. Provider engagement is critical to effective health plan-provider partnerships to overcome barriers, change behavior, and improve chronic disease care quality and population outcomes. This study was funded by Cigna. The manuscript was prepared as a work for hire. Hong, Esse, Gallardo, Serna, Fosshat, and Mamvou are employees of CareAllies, a Cigna company. Bruce was employed by Cigna at the time of the study. Vadhariya reports a past internship at Regeneron Pharmaceuticals, unrelated to this work. Abughosh reports grants from Regeneron Pharmaceuticals, Valeant Pharmaceuticals, Sanofi, and BMS/Pfizer, unrelated to this work.