BACKGROUND: There are limited data regarding national patterns of pharmacotherapy for atrial fibrillation (AF) among older patients. Drug exposure data are now captured for Medicare beneficiaries enrolled in prescription drug plans. OBJECTIVE: To describe pharmacotherapy for AF among Medicare beneficiaries. METHODS: By using a 5% national sample of Medicare claims data, we compared demographic characteristics, comorbidity, and treatment patterns according to Medicare Part D status among patients with prevalent AF in 2006 and 2007. RESULTS: In 2006, 27,174 patients (29.3%) with prevalent AF were enrolled in Medicare Part D. In 2007, enrollment increased to 45,711 (49.1%). Most enrollees were taking rate-control agents (74.0% in 2007). β-Blocker use was higher in those with concomitant AF and heart failure and increased with higher CHADS(2) scores (P <.001). Antiarrhythmic use was 18.7% in 2006 and 19.1% in 2007, with amiodarone accounting for more than 50%. Class Ic drugs were used in 3.2% of the patients in 2007. Warfarin use was <60% and declined with increasing stroke risk (P <.001). CONCLUSION: Pharmacotherapy for AF varied according to comorbidity and underlying risk. Amiodarone was the most commonly prescribed antiarrhythmic agent. Postmarketing surveillance using Medicare Part D claims data linked to clinical data may help inform comparative safety, effectiveness, and net clinical benefit of drug therapy for AF in older patients in real-world settings.