Serial coagulation studies were done in four women with acute fatty liver of pregnancy. All had coagulopathy, laboratory evidence of diffuse intravascular coagulation, and marked depletion of plasma antithrombin III. Two of these women had persistent intravascular coagulation for 4 days after delivery. The others had prompt control of intravascular coagulation coincident with elevation of the antithrombin III concentration by plasma transfusion. Severe antithrombin III depression may be a major cause of the persistent intravascular clotting and can be corrected by plasma transfusion.