We studied whether hemostatic abnormalities contribute to the increased risk of stroke in patients with nonvalvular atrial fibrillation. Hemostatic function was studied in four age-matched groups: 20 patients with nonvalvular atrial fibrillation and a previous ischemic stroke, 20 patients with nonvalvular atrial fibrillation without a previous stroke, 20 stroke patients with sinus rhythm, and 40 healthy controls. Both groups with nonvalvular atrial fibrillation had significantly higher concentrations of von Willebrand factor, factor VIII:C, fibrinogen, D-dimer (a fibrinolytic product), beta-thromboglobulin, and platelet factor 4; a significantly higher fibrinogen/antithrombin ratio; and significantly higher spontaneous amidolytic activity than the healthy controls. Prekallikrein levels were significantly lower in both groups with nonvalvular atrial fibrillation. Stroke patients with sinus rhythm had normal hemostatic function, normal concentrations of platelet-related factors, and a slightly increased concentration of fibrinopeptide A compared with the healthy controls. Both groups with nonvalvular atrial fibrillation differed from the stroke patients with sinus rhythm as they did from the healthy controls. No difference in hemostatic function was seen between the nonvalvular atrial fibrillation patients with and without a previous ischemic stroke. Thus, alterations in hemostatic function may contribute to the increased risk of stroke in patients with nonvalvular atrial fibrillation.