OBJECTIVE To determine whether the amount of p24 antigenemia in the first 6 months of life is a predictor of survival in children infected vertically with human immunodeficiency virus type 1. METHODS A retrospective study of vertically infected infants and children who were followed prospectively from early infancy and who had quantitation of plasma p24 antigen concentration in the first 6 months of life. Infants were first stratified by duration of survival as infants who died before 2 years of age (short-term survivors) and infants who survived to 2 years of age (intermediate-term survivors). The median p24 antigen concentration and the proportion of infants in each group with high concentrations of antigen were compared. Analyses with and excluding all p24 determinations made after the use of antiretroviral agents were compared Kaplan-Meier product limit analysis was used to compare survival in infants with low and high antigenemia during the first 6 months of life. RESULTS The median p24 antigen concentration in 15 short-term survivors was 228 pg/ml, compared with 14 pg/ml in 26 intermediate-term survivors (p < 0.05). The proportion of children with > 100 pg/ml of p24 was higher in short-term than in intermediate-term survivors (p = 0.01). Survival to 2 years of age in infants in whom all p24 antigen values during the first 6 months of life were 100 pg/ml or less was 91%, in comparison with 39% in infants with values greater than 100 pg/ml (p = 0.0017). CONCLUSIONS Elevated p24 antigenemia in the first 6 months of life is associated with shorter survival and may be a useful predictor of outcome.