A 38-year-old female with acute lymphoblastic leukemia developed monoplegia of the left upper extremity following chemotherapy for remission induction consisting of vincristine, prednisolone, cyclophosphamide, adriamycin and methotrexate. Hemorrhagic infarction due to thrombosis of the right cortical vein was diagnosed by brain images using computed tomography and magnetic resonance imaging in addition to the clinical course. At this time, a plasma level of fibrinogen measured by the thrombin time method had decreased to 84 mg/dl, while its antigenicity was 276 mg/dl. There was no evidence of activation of the blood coagulation and fibrinolysis system nor of abnormal liver function. A mixing test with normal plasma disclosed the absence of an inhibitory factor against fibrin polymerization in her plasma. Fibrinogen levels as assessed by the thrombin time method recovered to the antigenicity level one and half months later. The discrepancy between the activity and antigenicity of fibrinogen indicated the occurrence of acquired dysfibrinogenemia, probably induced by antileukemic agents. Thus, it is suggested that dysfibrinogenemia is a possible cause of cerebral thrombosis in this patient.