OBJECTIVES To compare medication adherence among patients exclusively receiving 30-day or 90-day supplies of maintenance medications from a single distribution channel. METHODS Patients (n=289,337) receiving maintenance medications with only a 30 or 90 day supply and from one distribution channel (retail/mail) for all their prescriptions were selected from a 10% random sample of members (n=22,577,122) with continuous pharmacy benefits eligibility from July 1, 2006 through December 31, 2007. Medication Possession Ratios (MPRs) were calculated for ACEs, ARBs, calcium channel blockers (CCBs), sulfonylureas, metformin, and statins for these patients; from their first pharmacy claim between January 1 and March 30, 2007 through December 31, 2007. Patient MPRs were aggregated by their 30/90-day supplies, retail/mail channel, and whether they were initiators (no claims from July 1, 2006 to December 31, 2006) or continuers. RESULTS Overall, MPRs ranged from 85% (metformin) to 88% (ACEs and CCBs) for individuals filling 90-day and 69% (metformin) to 76% (CCBs) for individuals filling 30-day supplies, with 90-day averaging 19.4% higher than 30-day. MPRs for 90-day mail were systematically higher than 90-day retail (average MPR: 82.4% vs. 80.9%), and member costs were less for 90-day mail in five therapeutic classes (average cost difference: $10.16 per prescription). Initiator MPR differences were more pronounced, ranging from 72% (statins and metformin) to 76% (ACEs) for 90-day and 52% (sulfonylureas and metformin) to 56% (statins) for 30-day supplies. Initiators who exclusively received 90-day mail prescriptions also benefited from systematically higher MPR scores (5 to 7 percentage points, on average) than those exclusively receiving 90-day retail prescriptions. CONCLUSIONS Extended days supply (i.e., 90-day) for maintenance medications substantially increased adherence measures, overall and among those initiating therapy. However, lower adherence among patients receiving 90-day retail prescriptions relative to those receiving 90-day mail, may be attributable to the higher out-of-pocket costs from prescriptions for current 90-day retail programs.