Risk factors for heterosexual HIV transmission are not fully understood. In fact, a proportion of people with sexual exposure to HIV remain uninfected despite multiple and continuous intercourse with HIV-infected partners. In this work, we have analyzed those virologic parameters potentially involved in the transmission of HIV through heterosexual contact. Thirty-eight couples with continuous unprotected sexual intercourse were included. HIV transmission occurred in 10 of 38 couples. No differences in clinical characteristics, exposure time, sexual practices, CD4 counts, or polymorphism in CCR5 were found between transmitter and nontransmitter groups. In contrast, virologic data were different between both groups; median values of viral load were 21.139 and 5.484 RNA copies/ml of plasma in the transmitter and nontransmitter groups, respectively, and a significant difference was found in mean viral load values (p = .03, Mann-Whitney test). Viral isolation was obtained in 90% of transmitters, but in only 44% of nontransmitter subjects (p = .02, Fisher's exact test). These data show that viral load levels and a positive viral isolation in culture must be considered as risk factors for heterosexual transmission of HIV-1.