Clinical trials during the past 20 years have revolutionized the antithrombotic management of atrial fibrillation. Based on consideration of 30 randomized trials involving 29,017 participants, adjusted-dose warfarin remains the most efficacious prophylaxis against stroke for atrial fibrillation patients at moderate-to-high risk (compared with antiplatelet agents, warfarin reduces stroke by about 40%). The optimal INR for prevention of stroke for most atrial fibrillation patients is probably 2.0–2.5; INRs of 1.6–1.9 provide substantial protection, 80–90% of that afforded by higher intensities. Warfarin-associated intracerebral hemorrhage is an increasing problem as more elderly patients with atrial fibrillation are anticoagulated. Modest reductions in blood pressure results in large decreases in this most dreaded complication of warfarin; anticoagulation of elderly atrial fibrillation patients should be accompanied by a firm commitment to control hypertension. Warfarin-associated intracerebral hemorrhage has a 50% early mortality. A wide range of acute treatments to urgently reverse anticoagulation have been recommended by experts, but prevention is a far better option than treatment of this devastating problem.