OBJECTIVE: To compare drug costs and adherence among Medicare beneficiaries with the standard Part D coverage gap versus supplemental gap coverage in 2006. DATA SOURCES: Pharmacy data from Medicare Advantage Prescription Drug (MAPD) plans. STUDY DESIGN: Parallel analyses comparing beneficiaries aged 65+ with diabetes in an integrated MAPD with a gap versus no gap (n=28,780); and in a network-model MAPD with a gap versus generic-only coverage during the gap (n=14,984). PRINCIPAL FINDINGS: Drug spending was 3 percent (95 percent confidence interval [CI]: 1-4 percent) and 4 percent (CI: 1-6 percent) lower among beneficiaries with a gap versus full or generic-only gap coverage, respectively. Out-of-pocket expenditures were 189 percent higher (CI: 185-193 percent) and adherence to three chronic drug classes was lower among those with a gap versus no gap (e.g., odds ratio=0.83, CI: 0.79-0.88, for oral diabetes drugs). Annual out-of-pocket spending was 14 percent higher (CI: 10-17 percent) for beneficiaries with a gap versus generic-only gap coverage, but levels of adherence were similar. CONCLUSIONS: Among Medicare beneficiaries with diabetes, having the Part D coverage gap resulted in lower total drug costs, but higher out-of-pocket spending and worse adherence compared with having no gap. Having generic-only coverage during the gap appeared to confer limited benefits compared with having no gap coverage.