The expanding indications for zidovudine treatment make it important to elucidate the safety and toxicity of this drug for pregnant women and their fetuses. We asked pediatricians and obstetricians at the AIDS (acquired immunodeficiency syndrome) Clinical Trials Units to report information about pregnant women infected with the human immunodeficiency virus who were continuing their pregnancies and had received, or were receiving, zidovudine during gestation. Reports of 43 women were received from 17 institutions. Doses of zidovudine ranged from 300 to 1200 mg per day, and 24 women took the drug for at least two trimesters. There were two reported instances of maternal toxicity (one gastrointestinal and one heatologic.) No teratogenic abnormalities occurred in the 12 infants with first-trimester exposure to zidovudine. All the infants, including two sets of twins, were born alive. The 38 singleton infants born at term for whom birth weights were reported had a mean birth weight of 3287±670 g; two cases of intrauterine growth retardation were reported among the infants delivered at term. Hemoglobin values, which were available for 31 newborns, ranged from 7.0 to 12.4 mmol per liter (11.2 to 20 g per deciliter); 3 of the 7 newborns with hemoglobin values of less than 8.4 mmol per liter (13.5 g per deciliter) were born prematurely. Zidovudine was well tolerated by the pregnant women and was apparently not associated with malformations in the newborns, premature birth, or fetal distress. No pattern of hematologic toxicity was observed in the newborns, but the anemia and growth retardation seen in a minority of the infants could, in part, have resulted from their mothers' treatment with zidovudine. (N Engl J Med 1992;326:857–61.).