SummaryQuantitative serum antibody to p24 was evaluated as a predictor of risk of vertical transmission of human immunodeficiency virus type 1 (HIV-1) infection. HIV-positive mothers, 13 with HIV-infected children and 24 with noninfected children were investigated during pregnancy and at the time of delivery. A statistically significant difference in anti-p24 titers was found between the mothers with infected and those with noninfected children independent of whether antibodies were measured during pregnancy or at the time of delivery. High anti-p24 levels correlated with a low risk of vertical transmission, whereas low anti-p24 titers were associated with an increased risk of vertical transmission. Although the number of CD4+T-cells was lower and neopterin and beta-2 microglobulin values were higher in the group of mothers with infected children than in the noninfected group, no statistical significance was achieved due to the small sample size.