Eighteen patients were given carefully controlled intravenous heparin for thromboembolic disease of pregnancy. Thirteen were treated in later gestation and postpartum, mainly following warfarin therapy. One was treated in early pregnancy and 4 in the postpartum period. Heparin was delivered by constant infusion and dosage regulated by maintaining the thrombin time at 2 to 4 times normal. Therapy was stopped for a few hours before and after delivery so that the thrombin time was normal or nearly normal at birth. Two minor episodes of thrombosis recurred in 1 patient while on warfarin, and significant postpartum bleeding occurred in one other. Substituting heparin for warfarin in late gestation was of value in preventing neonatal bleeding complications.