Cord blood was anonymously screened to determine the prevalence of human immunodeficiency virus (HIV) seropositivity in neonates admitted to the neonatal intensive care unit (NICU) at the Bronx Lebanon Hospital Center, located in the South Bronx. We speculated that factors leading to admission to the NICU such as low birth weight, prematurity and being small for gestational age would also be associated with an increased prevalence of HIV seropositivity. During the study period the prevalence of HIV seropositivity was 11.6% in the NICU population. There was no significant difference in maternal age, gravidity, race and sex in HIV-seropositive vs. HIV-seronegative newborns. There was a significantly increased incidence of maternal drug use (P less than 0.01), babies small for gestational age (P less than 0.005) and microcephaly (P less than 0.02) in seropositive vs. seronegative NICU babies. The results of this study suggest that the NICU population may comprise a significant number of infants of HIV-infected mothers.