The changing spectrum of herpes simplex virus type 1 (HSV-1) and herpes simplex virus type 2 (HSV-2) infections makes it important to define the seroepidemiology of HSV. The object of this study was to determine the prevalence of HSV-1 and HSV-2 immunoglobulin G antibodies in a young Swedish population by investigating 2106 serum samples from people aged 0-19 y. Sera were tested in HSV type-specific enzyme-linked immunosorbent assays using glycoprotein G-1 (gG-1) and glycoprotein G-2 (gG-2) as antigens. The overall seroprevalence was 31% (95% CI 29-33) for HSV-1 and 0.5% (95% CI 0.2-0.9) for HSV-2. The HSV-1 seroprevalence was higher with increasing age, and significantly higher in the age cohort 15-19 y compared with 1-4-y-olds (37% vs 24%). The HSV-1 infection seemed to be acquired early in life. In the age cohort 1-2 y, the prevalence was over 20%, presumably reflecting an established viral infection. In adolescence the HSV-1 seroprevalence may reflect both oral and sexual transmission. The seroprevalence in the oldest age cohort did not differ significantly from that seen in a Swedish study in which sera were sampled from young girls in the 1970s.