Thromboembolism is an important complication of heart failure. To test the hypothesis that heart failure may be associated with hemostatic dysfunction, we studied hemostatic function in 21 patients with stable chronic heart failure and related these measures to the severity of heart failure as assessed by clinical evaluation, neuroendocrine activation, radionuclide ventriculography, and cardiopulmonary exercise testing. Plasma and blood viscosity were elevated; all patients showed evidence of platelet activation, and many had elevated plasma concentrations of fibrinopeptide A, D-dimer, and von Willebrand factor. The plasma concentrations of these variables were poorly interrelated and related poorly to the severity of heart failure. Plasma concentrations of angiotensin II and endothelin were correlated, and the latter was also correlated with the plasma concentration of von Willebrand factor. Patients with chronic heart failure have hemostatic abnormalities that may predispose them to thromboembolic events and may be in part due to neuroendocrine activation.