This article examines the effects of girls' early marriage on their risk of acquiring HIV/AIDS. By comparing several underlying HIV risk factors, it explores the counterintuitive finding that married adolescent girls in urban centers in Kenya and Zambia have higher rates of HIV infection than do sexually active unmarried girls. In both countries, we find that early marriage increases coital frequency, decreases condom use, and virtually eliminates girls' ability to abstain from sex. Moreover, husbands of married girls are about three times more likely to be HIV-positive than are boyfriends of single girls. Although married girls are less likely than single girls to have multiple partners, this protective behavior may be outweighed by their greater exposure via unprotected sex with partners who have higher rates of infection. These results challenge commonly held assumptions about sex within marriage.