Four methods of culturing human immunodeficiency virus type 1 (HIV-1) from peripheral blood mononuclear cells and two serum antigen tests were assessed as predictors of infection status in children born to HIV-1-infected mothers. Of 36 infants whose cocultures were quantitative, all 15 who were deemed to be infected with HIV-1 (nine with symptoms, six without symptoms) by clinical criteria or persistence of Western blot reactive antibody had positive culture results, and all 21 uninfected seroreverters had negative culture results (sensitivity = 100%; specificity = 100%). Quantitative coculture was more sensitive than a technique in which cells were counted and stimulated with phytohemagglutin but not cocultivated with cells from seronegative donors, and more sensitive than two other qualitative techniques evaluated in samples from 80 children, in which cells were not enumerated before culture. The level of leukoviremia in children with symptoms did not differ appreciably from the level of leukoviremia in symptom-free infected children. Among those with positive results on quantitative coculture, only 40% also had free HIV-1 antigen in serum, whereas 86% had antigen in immune complexes. Among the methods evaluated, quantitative HIV-1 coculture was the best indicator of infection status in children.