OBJECTIVE To examine relative medication adherence of patients filling 90-day supplies of maintenance medications using retail and mail order channels. It was hypothesized that adherence rates would not differ across the 2 channels. METHODS A cross-sectional retrospective analysis was conducted using de-identified pharmacy claims data from a large pharmacy benefit manager (PBM) database over a 2-year period (January 2008 to August 2010). Patients who were continuously eligible for at least 12 months during this time frame, with benefit plan designs that allowed filling of 90-day supplies either at retail or by mail order pharmacy, were selected. METHODS Adherence was measured by medication possession ratio (MPR) within a 1-year period. Propensity score matching was employed to minimize differences between the Retail-90 group and Mail Order-90 group. RESULTS Overall, patients filling 90-day prescriptions for 9 therapeutic groups (antiasthmatics and bronchodilators, antidepressants, antidiabetics, antihyperlipidemics, antihypertensives, beta blockers, calcium channel blockers, diuretics, and thyroid agents) at retail pharmacies demonstrated a propensity score–matched average MPR that was statistically higher than for patients filling prescriptions via mail order (77.0% vs 76.0%). There were no significant differences in MPR (post-matching) between 90-day retail and mail order channels for individual therapeutic groups, except for antidiabetics (80.2% vs 83.1%). CONCLUSIONS On a propensity-matched basis, patients who fill maintenance prescriptions at retail have a slightly, but statistically significantly, higher MPR than patients who fill their prescriptions by mail