This study sought to identify genital tract characteristics associated with vertical transmission of human immunodeficiency virus type 1 (HIV-1). HIV-1 DNA and RNA, HIV-1 env diversity, and inflammatory cells were quantified in cervicovaginal lavages (CVLs) of 24 women enrolled in the Women and Infants Transmission Study; 7 women transmitted HIV-1 perinatally. Vaginal candidiasis, HIV-1 culture positivity, levels of HIV-1 DNA and cell-free RNA, and HIV-1 env diversity were significantly higher in the CVLs of transmitters. CVL HIV-1 DNA levels correlated with higher levels of inflammatory cells and cell-free HIV-1 RNA. Of subjects with paired blood and CVL specimens, there was more HIV-1 env heterogeneity between blood and CVLs in transmitters than in nontransmitters. In summary, increased HIV-1 shedding is correlated with a more complex population of HIV-1 quasispecies in the genital tracts of parturient women, which may increase the probability that a fetotropic strain is transmitted.