A familial abnormal antithrombin III (AT-III) is reported. The characteristic of the abnormality in this family is low heparin cofactor activity with normal progressive antithrombin activity and normal or rather increased level of AT-III antigen. The patient is a 23-year-old female who had suffered from recurrent thrombophlebitis involving her lower extremities. Her plasma AT-III antigen concentration was 54 mg/dl and progressive antithrombin and factor Xa inhibitory activities were of normal level. However, the heparin cofactor activity of her plasma was as low as 26% of normal control. On crossed immunoelectrophoresis (CIE) containing heparin in the first dimension agarose, patient's AT-III showed no increase in electrophoretic mobility compared to that in the absence of heparin, suggesting that the patient's AT-III has no affinity for heparin. From CIE pattern in the presence of heparin, the patient was found to be a homozygote, and parents and one of her younger sisters were heterozygotes. Thus, the mode of inheritance is proposed to be autosomal dominant.