A child presented with the classical clinical and hematologic findings of purpura fulminans, appearing after a mild attack of varicella. His blood proved entirely incoagulable, and the plasma apparently contained free thrombin. Treatment consisted of intravenously administered heparin sodium, as well as prednisone and transfusions. His response to therapy was monitored by assay of clottable fibrinogen, by the "P and P" test of Ware and Stragnell, and the platelet count. He recovered completely, and remains well five years later.