Twenty-six patients with fulminant hepatic failure were treated with daily infusions of antithrombin III concentrate until recovery of consciousness or death. Seven patients were alive (group A), 7 survived 17 to 47 days after treatment (group B), and 12 died within 9 days (group C). Decreased plasma antithrombin III levels increased on the day after treatment, irrespective of the pretreatment levels in all patients. Continuous or temporary normalization was seen in all patients in groups A and B, but in only 5 in group C patients whose bleeding was extensive (p less than 0.05). An abrupt drop in peripheral platelet counts occurred when plasma antithrombin III levels were below normal. General bleeding accompanied this drop. These results suggest that maintained normal plasma antithrombin III levels are beneficial for prolonged survival time in fulminant hepatic failure, probably through controlling intravascular coagulation, and that antithrombin III infusion may be useful for such treatment.