Homozygous protein C deficiency or homozygous protein S deficiency are rare genetic diseases with catastrophic and fatal purpura fulminans-like or thrombotic complications occurring during the neonatal period. These diseases can now be successfully treated. Purpura fulminans is at least in part a cutaneous manifestation of the syndrome of systemic DIC. It is characterized by microvascular thrombosis in the dermis followed by perivascular hemorrhage, necrosis, and minimal inflammation. Laboratory findings are consistent with DIC. Although the pathogenesis is not fully understood, the DIC in purpura fulminans appears to involve the skin selectively. The development of purpura fulminans from homozygous protein C or protein S deficiencies can be separated into the two distinct phases. The first phase is the time period when the initial reversible lesions develop and grow. This reversible progression can be halted and reversed with the administration of protein C or protein S. The second phase is the irreversible stage in which the lesion continues to develop into a necrotic lesion, whether or not treated with protein C. This irreversible lesion will ultimately develop into a large full-thickness necrotic injury of the skin. It is very similar to the lesions seen in idiopathic purpura fulminans, warfarin-induced skin necrosis, and acute infectious purpura fulminans. Unfortunately, our current understanding of the mechanism or mechanisms of the induction and propagation of the purpura fulminans-like lesions in homozygous protein C or protein S deficiencies is minimal, since it has never been studied. We can only speculate on the mechanism based on laboratory data and comparison with the little that is known about the other similar types of lesions.