The objective of this study was to evaluate whether the change in sexual behavior among homosexual men observed after the start of the acquired immunodeficiency syndrome epidemic resulted in a change in herpes simplex virus (HSV) seroprevalence in this group over time. In a cross-sectional study, the prevalence of herpesvirus types 1 (HSV1) and 2 (HSV2) was determined at study entry in 1984-1985 and 1995-1997 among 532 young (aged < or = 30 years) homosexual men participating in the Amsterdam Cohort Studies on HIV/AIDS. Risk factors for the presence of HSV antibodies, including human immunodeficiency virus infection, were evaluated, and their influence on HSV prevalence over time was assessed. A strong decrease in HSV1 and HSV2 seroprevalence, from 80.6% to 59.0% and from 51.3% to 19.0%, respectively, was observed between the two time periods. This decrease was not markedly influenced by various demographic and socioeconomic factors. After data were controlled for several markers of sexual activity (such as number of sex partners, human immunodeficiency virus infection, and past episode(s) of gonorrhea), it appeared that the decline in HSV seroprevalence was explained by a concurrent decrease in the presence of these markers. The authors conclude that among young homosexual men in this study, the strong decrease in HSV seroprevalence was associated with a concurrent shift in sexual behavior. Furthermore, these data suggest an increasing sexual component in HSV1 transmission over time.