A family in which there were two certain and three possible cases of thrombotic microangiopathy in two generations is presented. All afflicted members studied presented with acute renal failure, and accelerated hypertension. No abnormal platelet-aggregating activity could be identified in the plasma of asymptomatic family members or in the surviving patient in remission. Although an increase in the Factor VIII: von Willebrand's factor level was found in the sole surviving patient, the multimer pattern was normal. Platelet-associated IgG level was also normal in this individual. Thus, individuals predisposed to this familial form of thrombotic microangiopathy do not have a demonstrable marker which can be implicated in the development of the disorder. Long-term study of this kindred is required to determine the factors which are important in the pathogenesis of thrombotic microangiopathy in the affected individuals.