Antithrombin III (AT III) is the main physiological inhibitor of blood coagulation. In a prospective study, plasma AT III was determined in 653 women during pregnancy, using an automated amidolytic technique. A control value 8 weeks after delivery was obtained in 192 of the women. In women with pregnancy-induced or aggravated hypertension a significant decrease in AT III levels was observed compared with normotensive controls of the same period of gestation and compared with the patients' own control values 6-8 weeks after delivery. No AT III depression occurred in patients with chronic hypertension during pregnancy. Patients with pregnancy hypertension and proteinuria had lower AT III levels than those without proteinuria, whose AT III levels were also depressed. Lowest AT III levels were seen in 2 eclamptic patients and in patients with severe preeclampsia, whose pregnancies were terminated for fetal distress while the infants were still preterm. Monitoring At III levels is of value in preeclampsia.