The relation between the heart lesions of thrombotic thrombocytopenic purpura and clinical cardiac dysfunction was studied in 17 autopsied patients. Thirteen of the 17 patients had extensive small-vessel thromboses and in some instances hemorrhages and focal necroses within the heart. Congestive heart failure was present in nine of the 17 patients. Thrombotic microcirculatory cardiac lesions and anemia-related high cardiac output may have contributed to cardiac dysfunction. Serial histologic sections of the cardiac conducting system in 10 patients showed microthrombi in seven and associated hemorrhages in five. Lesions were localized to the atrioventricular node and His bundle parts of the system with sparing of the sinoatrial node and bundle branches. Two patients had electrocardiographic evidence suggesting lesions within conducting tissues. Thrombi and hemorrhages are common findings in the conducting tissues in thrombotic thrombocytopenic purpura and may account for cardiac arrest or transient rhythm disturbances.