The incidence of thromboembolic complications (TE) during pregnancy in women with congenital antithrombin III (AT) deficiency has retrospectively been estimated to be about 70%. 8 women with congenital AT deficiency were studied during 9 pregnancies. Subcutaneous or intravenous heparin in doses to prolong the activated partial thromboplastin time (APTT) was given during pregnancy as prophylaxis or therapeutic treatment. During delivery and abortion the AT level was brought to normal by infusion of AT concentrate and the heparin was reduced or withdrawn. Four pregnancies were uncomplicated with regard to TE and resulted in 4 healthy children. Five pregnancies were terminated by induced or spontaneous abortion. 1 woman had TE during heparin prophylaxis and 2 women had TE before the prophylaxis was started. 1 of the latter suffered from a new TE during continued heparing treatment. Insufficient prolongation of APTT was registered at the time of TE in both women with TE during heparin treatment.