Fourteen patients were treated with anticoagulants during 15 pregnancies. Eleven had venous thrombosis, three with pulmonary embolism; three had prosthetic heart valves; and one had mitral stenosis with pulmonary hypertension. All 15 were treated with heparin and 10 with warfarin. The mean duration of heparin therapy was 4.5 weeks and of warfarin 14 weeks. There were two minor episodes of recurrent thromboembolism and three minor haemorrhagic episodes during anticoagulant therapy. No fetal or neonatal complications occurred.There appears to be no ideal anticoagulant regimen for treating thromboembolic disease in pregnancy. A compromise approach based on experimental and clinical findings is suggested, which is considered to offer reasonable protection to the mother without undue risk to the fetus. This is heparin when an anticoagulant is indicated in the first trimester, after 37 weeks' gestation, and for the treatment of the initial thromboembolic episodes. Oral anticoagulants are used at other times when long-term anticoagulant therapy is indicated.