Thirty patients have been treated with heparin during pregnancy or in the immediate postpartum period. Twenty had severe preclamptic toxemia. Five presented before 30 weeks and in addition had low estriol levels and high blood ureas. The chance of a successful pregnancy in such patients is very low indeed; however 3 of these 5 patients had living babies. Even in patients presenting after 30 weeks of gestation, if severe preeclampsia is accompanied by low estriol levels and high blood urea levels, the anticipated fetal survival is poor. However, all 5 patients in this group were treated with heparin and had living babies. In the small group of patients with glomerulonephritis, there was a suggestion that heparin treatment may have been of value. Biopsy studies suggested that heparin may help to prevent lesions in arteries and arterioles which have been noted to develop in association with preeclamptic toxemia. In four patients treated in the postpartum period, there was possible benefit in terms of renal function, intravascular coagulation, vessel lesions, and subsequent successful pregnancies.