The seroepidemiology of infection due to herpes simplex type 1 (HSV-1) and varicella-zoster (VZV) viruses was investigated in 224 Saudi children aged from under 1 year to 15 years and 452 adults (healthy male blood donors and pregnant women) using a presently available sensitive indirect immunofluorescence technique to detect antibodies to these viruses in order to determine the age of primary infection. Age-specific prevalence of IgG antibodies to HSV-1 and VZV showed a progressive increase with age in both males and females with no obvious sex-related variation in the level. The overall prevalence of antibodies was 60 per cent for HSV-1 and 68 per cent for VZV in children whereas about 90 per cent of the adults showed the presence of antibodies to both viruses. Virological and serological confirmation of two cases in children of herpes simplex encephalitis (HSE) due to HSV-1 and VZV reactivation in two adults is described.