Four methods of estimating mother-to-child transmission rates of human immunodeficiency virus type 1 (HIV-1), based on the 1992 Ghent workshop, were compared in a multicenter New York City prospective cohort study in 1986-1992. Of 833 infants born to women at risk of HIV-1 infection, 388 were born HIV-1 seropositive and 445 were HIV-1 seronegative. The four methods, the Antibody Only, Indirect, Direct, and Virologic Methods, yielded transmission rate estimates of 19-25%, classifying 59-89% of the cohort. Estimation based on persistence of HIV-1 antibody and clinical assessment yielded transmission rates similar to those methods that incorporated virologic testing.