A number of clinical trials have shown the value of anticoagulating patients with nonrheumatic atrial fibrillation to prevent ischemic stroke. The purpose of this study was to assess the cost-effectiveness of anticoagulation in nonrheumatic atrial fibrillation with particular reference to the very elderly (aged >75 years) who have a higher incidence of bleeding events while undergoing anticoagulation. We calculated the incremental costs per life-year gained for 4 base cases using efficacy data from the Boston Area Anticoagulation Trial for Atrial Fibrillation, the meta-analysis of the 5 nonrheumatic atrial fibrillation trials, cost data from a district general hospital, and review of the literature. The cost per life-year gained free from stroke over 10 years ranged from -pound sterling 400.45 (ie, a resource saving achieved for each life-year gained free from stroke) to pound sterling 13,221.29. The results were most sensitive to alteration in the frequency of anticoagulation monitoring. For medical and economic reasons, anticoagulation treatment in the prevention of ischemic stroke is justified. Although older patients are more at risk of adverse events, anticoagulation is more cost-effective in this group.