We determined the prevalence of herpes simplex virus (HSV) and cytomegalovirus (CMV) antibodies in a cohort of adolescents 12 to 22 years of age in anticipation of the development of vaccines to control HSV and CMV infections. For the overall study population, we found that 62% were seropositive for HSV type 1 (HSV-1), 12% were seropositive for HSV type 2 (HSV-2), and 65% were seropositive for CMV. Race was not related to HSV-1 seropositivity, but African-American adolescents were more likely than Caucasian adolescents to be seropositive for HSV-2 and CMV. Girls also were more likely than boys to be seropositive for HSV-2 and CMV. For boys, history of a sexually transmitted disease was identified as a risk factor for HSV-2 seropositivity; for girls, a greater number of sexual partners increased the risk of being seropositive for HSV-2. Our data demonstrating a high prevalence of infection during adolescence suggest that immunization for HSV-1, HSV-2, and CMV may need to occur in childhood.