Synchronizing medication refills-renewing all medications at the same time from the same pharmacy-is an increasingly popular strategy to improve adherence to medication regimens, but there has been little research regarding its effectiveness. In light of increasing policy interest, we evaluated the impact of a pilot refill synchronization program implemented by a large national insurer. A random sample of Medicare Advantage patients receiving mail-order refills for common maintenance medications (antihypertensive, lipid-lowering, or antidiabetic agents) were invited to join the program and followed for twelve months. On average, the absolute increase in the proportion of patients deemed adherent during follow-up was 3-10 percentage points for the intervention group, compared to 1-5 percentage points for the control group. Patients with poorer baseline adherence showed larger increases in the absolute proportion deemed adherent in intervention (23-26 percentage points) compared to a control group (13-15 percentage points). Synchronizing refills might be a promising intervention to improve adherence to maintenance medications, especially among Medicare patients with low baseline adherence.