Patients are enrolled during their pregnancy, between 14 and 34 weeks of gestation. They are chosen by random selection to receive AZT or placebo. Treatment continues until labor at which time they begin to receive continuous intravenous study drug. Study drug treatment is discontinued after the umbilical cord is clamped. AZT is then offered all women as per labeled indications for 6 weeks postpartum, while appropriate medical followup is being arranged. Mothers who develop an AIDS defining illness or whose CD4+ cell counts decrease to less than 200 cells/mm3 during pregnancy are offered open-label drug at that time. The mother is followed by her primary obstetrician at an AIDS Clinical Trials Unit (ACTU) or subunit facility. The mother may deliver at the ACTU or a non-ACTU site. Treatment of the infant is started in the newborn nursery and continues on an outpatient basis. Infants receive the same study treatment as the mother for 6 weeks, and are monitored to week 78.