• Perinatal transmission of human immunodeficiency virus is thought to occur in 25% to 50% of the offspring of infected women. Standard diagnostic methods do not permit identification of the infected newborns. To assess diagnostic methods and document the natural history of perinatal human immunodeficiency virus infection, 20 children born to human immunodeficiency virus–infected women were followed prospectively for 18 months by measuring antibody titer, Western blot profiles, and antigenemia, and the results were compared with clinical outcome. Endogenous synthesis of anti–human immunodeficiency virus IgG was demonstrated in 6 of the 8 infected children. Four children synthesized IgM against human immunodeficiency virus. Five had demonstrable p24 antigenemia. No significant differences between infected and noninfected children were noted at birth except drug withdrawal, which occurred more frequently in noninfected infants. The incidence of adenopathy, hepatomegaly, and neurologic and immunologic abnormalities in the infected children were compared with noninfected children. The distinguishing illnesses were the opportunistic infections, lobar pneumonia, and failure to thrive. Seven of the 8 infected children had human immunodeficiency virus–mediated disease by 1 year of age (Centers for Disease Control [Atlanta, Ga] P2 classification), and four had acquired immunodeficiency syndrome (Centers for Disease Control P2D). These studies offer an approach to diagnosis of human immunodeficiency virus infection in infants and document the natural history and possible outcomes of infected children.