In HIV-1 infection, the early set-point viral load strongly predicts both viral transmission and disease progression. The factors responsible for the wide spectrum of set-point viral loads are complex and likely reflect an interplay between the transmitted virus and genetically defined factors in both the transmitting source partner and the seroconverter. Indeed, analysis of 195 transmission pairs from Lusaka, Zambia, revealed that the viral loads in transmitting source partners contributed only~2% of the variance in early set-point viral loads of seroconverters (P=0.046 by univariable analysis). In multivariable models, early setpoint viral loads in seroconverting partners were a complex function of (i) the viral load in the source partner, (ii) the gender of the seroconverter, (iii) specific HLA class I alleles in the newly infected partner, and (iv) sharing of HLA-I alleles between partners in a transmission pair. Each of these factors significantly and independently contributed to the set-point viral load inthe newly infected partner, accounting for up to 37% of the variance observed and suggesting that many factors operate in concert to define the early virological phenotype in HIV-1 infection. © 2013, American Society for Microbiology.