Twenty-three children vertically infected with human immunodeficiency virus type 1 (HIV-1) were studied for viremia during the first days of life. Nine had HIV-1 infection within the first week (early); 14 had HIV-1 first detected by day 11-90 (late). The groups had similar incidence and time of onset of symptomatic HIV-1 infection and survival. CD4 T cell percentages, rates of CD4 T cell attrition, quantitative cell-associated viremia, and p24 antigen concentrations were comparable. Children with peak antigen concentrations >100 pg/mL during the first 6 months (5 early, 6 late) fared worse than those with lower p24 levels. Thus, HIV-1-infected infants with detectable virus in the first few days of life do not have a worse prognosis than infants whose virus is detectable only later. Elevated p24 antigenemia during the first 6 months of life correlates strongly with poor clinical outcome and is independent of the time virus was first detected.