Dynamic fluctuations of biological activities and antigen amounts of antithrombin III (AT-III) in acute cerebrovascular diseases were analyzed and evaluated. (1) AT-III levels assayed by both biological and immunological methods showed a gradual decrease with increasing age, and levels in patients with cerebral thrombosis were lower than those in patients with cerebral hemorrhage. (2) AT-III in cerebral thrombosis decreased slowly after administration of urokinase. After administration of aspirin (acetylsalicylic acid), however, AT-III levels increased presumably due to inhibition of coagulation activities. (3) Both biological activities and antigen amounts of AT-III increased after administration of an AT-III preparation, but the measured value, especially that of biological activity, was dissociated from the expected one, due to changes of the coagulation system after an operation.