Thirty-three infants born to human immunodeficiency virus type 1 (HIV-1)-seropositive women were evaluated from birth using plasma and peripheral blood mononuclear cell (PBMC) cultures, polymerase chain reaction, and serum p24 antigen assays. Five children were identified as infected. Evidence of infection was found in cord blood and subsequent samples from 2 infected children, suggesting in utero infection. Virologic studies on cord blood and early neonatal specimens from the 3 other infected children were negative but became positive by 8 weeks of age, suggesting either intrapartum transmission or sequestration of virus with subsequent detection. An increase in blood (plasma and PBMC) virus titers consistent with primary viremia was observed in 4 infants between 3 and 16 weeks of age. Blood virus titers subsequently declined in the absence of antiretroviral therapy and in the absence of activated HIV-1-specific cytotoxic T lymphocyte responses or broadly neutralizing antibodies. Confirmation of these results in larger studies may be helpful in the design of clinical trials to interrupt vertical transmission or to modify the course of infection.