We examined associations between enrollment in Medicare Part D pharmacy benefits and changes in medication acquisition from Department of Veterans Affairs (VA) pharmacies. We included all women and a random 10% sample of men who were VA enrollees, ≥65 years old as of January 1, 2004, and alive through December 2007. We used difference-in-differences models with propensity score weighting to examine changes in medication acquisition between 2005 (before Part D was implemented) and 2007 (after Part D implementation) for veterans who were or were not Part D enrolled. Of 231,716 veterans meeting inclusion criteria, 49,881 (21.5%) were enrolled. While 30-day medication supplies decreased from 26.2 to 23.4 for enrolled veterans, they increased from 36.6 to 37.4 for nonenrolled veterans (difference-in-differences: –4.0, p < .001). Reductions in 30-day supplies were greater among veterans who were required to pay VA copayments for some or all medications and who used VA and Medicare outpatient services.