To examine perinatal transmission of the human immunodeficiency virus type 1 (HIV-1) in Zaire, we screened 8108 women who gave birth at one of two Kinshasa hospitals that serve populations of markedly different socioeconomic status. For up to one year, we followed the 475 infants of the 466 seropositive women (5.8 percent of those screened) and the 616 infants of 606 seronegative women matched for age, parity, and hospital. On the basis of clinical criteria, 85 of the seropositive women (18 percent) had the acquired immunodeficiency syndrome (AIDS). The infants of seropositive mothers, as compared with those of seronegative mothers, were more frequently premature, had lower birth weights, and had a higher death rate in the first 28 days (6.2 vs. 1.2 percent; P less than 0.0001). The patterns were similar at the two hospitals. Twenty-one percent of the cultures for HIV-1 of 92 randomly selected cord-blood samples from infants of seropositive women were positive. T4-cell counts were performed in 37 seropositive women, and cord blood from their infants was cultured. The cultures were positive in the infants of 6 of the 18 women with antepartum T4 counts of 400 or fewer cells per cubic millimeter, as compared with none of the infants of the 19 women with more than 400 T4 cells per cubic millimeter (P = 0.02). One year later, 21 percent of the infants of the seropositive mothers had died as compared with 3.8 percent of the control infants (P less than 0.001), and 7.9 percent of their surviving infants had AIDS. We conclude that the mortality rates among children of seropositive mothers are high regardless of socioeconomic status, and that perinatal transmission of HIV-1 has a major adverse effect on infant survival in Kinshasa.