We analyzed the association between mother-to-child transmission (MTCT) of human immunodeficiency virus type 1 (HIV-1) and maternal neutralizing antibodies to heterologous primary isolates of various HIV-1 clades, to test the hypothesis that protective antibodies are those with broad neutralizing activity. Our study sample included 90 Thai women for whom the timing of HIV-1 transmission (in utero or intrapartum) was known. The statistical analysis included a conditional logistic-regression model to control for both plasma viral load and duration of zidovudine prophylaxis. The higher the titer of neutralizing antibodies to a heterologous strain of the same clade, the lower the rate of MTCT of HIV-1. More specifically, high levels of neutralizing antibodies to the MBA (CRF01_AE) strain were associated with low intrapartum transmission of HIV-1. This suggested that such heterologous neutralizing antibodies may be involved in the natural prevention of late perinatal HIV transmission. These data are consistent with the hypothesis that the use of some antibodies might help to prevent perinatal HIV transmission, through passive immunoprophylaxis. Moreover, the study of humoral factors associated with MTCT of HIV-1 may identify correlates of protection that should help in the design of efficient HIV/acquired immunodeficiency syndrome vaccines.