We sought to determine whether the detectability of phytohemagglutinin-inducible p24 (PHA-p24) in short term cultures of peripheral blood mononuclear cells correlates with an increased risk of vertical transmission among human immunodeficiency virus type 1 (HIV-1)-infected pregnant women and more severe symptomatology among HIV-1-infected infants. The assay for PHA-p24 was performed on specimens obtained from HIV-1-infected women during their pregnancy and from infants during the first 6 months of life. Infants were followed prospecitively to determine HIV-1 infection outcome and symptomatology. Among PHA-p24 positive women 9 of 19 (47.4%) gave birth to HIV-1-infected infants compared with 4 of 25 (16.0%) of PHA-p24-negative women (P= 0.02). Among women who tested PHA-p24-positive and had a CD4+lymphocyte count <500 cells/mm3, 8 of 15 (53.3%) gave birth to HIV-1-infected infants compared with 4 of 26 (15.4%) not meeting these conditions (P= 0.01). Among HIV-1-infected infants 4 of 5 (80%) of those testing PHA-p24-positive by one month of age developed an opportunistic infection or encephalopathy by 12 months of age, compared with none of the 11 infants testing PHA-p24-negative (P= 0.003). We conclude that PHA-p24-negative (P= 0.003). We conclude that PHA-p24 may be a usefulin vitromeasure for increased risk of vertical transmission among HIV-1-infected pregant women and increased risk for rapid development of severe disease among HIV-1-infected infants.