Patients with epidemic infections caused by Neisseria meningitidis serogroup C were studied to assess the relationship of abnormal coagulation parameters to prognosis. Patients were categorized into stages within the first hour of observation according to severity of illness. During the epidemic years 1986 through 1991, 113 patients with bacteriologically proven N. meningitidis infection were observed, 15 of whom died. Purpura fulminans was seen in 28 patients, of whom 14 (50%) died. Among the 14 surviving patients who had purpura fulminans, 10 suffered gangrene with deforming autoamputation secondary to the dermal microvascular thrombosis and hemorrhagic necrosis. Evaluation of the induced diffuse intravascular coagulation in 59 patients included studies of the naturally occurring anticoagulants, focusing on protein C and protein S. The magnitude of the declining levels of protein C, the degree of thrombocytopenia, and the presence of fibrin split products are directly related to the clinical severity of the illness (P = .0053). Thus, in individuals with severe disease expression, the risk of purpura fulminans with death or deformity was significantly increased when the platelet count was 3 (P = .0001) and protein C levels were low (P = .0158). The immaturity of the protein C system in children who are <4 years of age may contribute to the rapid and more frequent pathogenesis of purpura fulminans. Therapy directed at replacement of the naturally occurring anticoagulants, such as protein C, may ultimately improve the prognosis for individuals with purpura fulminans.