This study analyzed prescription drug utilization data from a Medicare drug discount card program and employer-insured plans to estimate the extent to which seniors filled prescriptions in Canada prior to Medicare Part D. The authors found that those living closer to the Canadian border had lower rates of filling prescriptions domestically, suggesting some crossed the border for cheaper drugs. The effect was greater for brand drugs and those without insurance coverage. Living 10% closer to the border was associated with 1.15% and 0.80% fewer generic and brand prescriptions filled domestically. The results indicate that while Medicare Part D likely reduced cross-border drug purchases, some seniors may still obtain medications from Canada to save on costs.