To confirm the pathogenesis and the genetic background of neonatal-onset purpura fulminans, two unrelated infants with this rare thrombotic syndrome and 47 of their asymptomatic relatives were studied. In both families, 27 subjects with hereditary partial deficiency of protein C, including both parents of each patient, were identified. The patient in whom it was possible to evaluate protein C directly showed no detectable levels of this plasma component. These findings confirm the linkage of neonatal purpura fulminans to a genetic trait with established mendelian transmission and strongly suggest that the syndrome is an expression of homozygosity for protein C deficiency. The dramatic clinical picture and the type of pathologic change that develops as a result of the lack of circulating protein C emphasize the vital role of this protein in protection from thrombin generation, mainly within the microvascular system. However, our data do not contribute to the evidence that partial familial protein C deficiency is associated with a major risk of venous thromboembolism.