The seroprevalence of herpes simplex virus type 2 (HSV-2) infection was studied among 4128 patients from sexually transmitted disease (STD) clinics who were enrolled in a randomized controlled trial of human immunodeficiency virus and STD counseling efficacy. HSV-2 seroprevalence was 40.8% and was higher in women than in men (52.0% vs. 32.4%; P<.0001) and higher in blacks than in nonblacks (48.1% vs. 29.6%; P<.0001). Among 14-19-year-old patients, 36.8% of black women and 25.8% of nonblack women were infected with HSV-2. Independent predictors of HSV-2 seropositivity included female sex, black race, older age, less education, more lifetime sex partners, prior diagnosis of syphilis or gonorrhea, and lack of HSV-1 antibody. The majority of HSV-2-seropositive persons (84.7%) had never received a diagnosis of genital herpes. HSV-2 infection is common in STD clinic attendees in the United States, even among young age groups, especially among women. Efforts to prevent genital herpes should begin at an early age. The high rate of undiagnosed HSV-2 infection likely contributes to ongoing transmission.