A patient with congenital antithrombin III deficiency and a history of thromboembolic events prior to pregnancy was successfully treated with subcutaneous heparin alone throughout two pregnancies. Although adjuvant antithrombin III infusions may have a role in the treatment of this disorder, especially in the peripartum period, there is no evidence that this approach is superior to heparin therapy alone. We present this case as further evidence that heparin therapy alone appears to be a safe and effective treatment for pregnant patients with this rare but serious disorder.