A seroepidemiologic survey on the prevalence of antibodies to Epstein-Barr virus (EBV) in Denmark was carried out. Sera from 518 children and 178 adults were investigated for IgG antibodies to the viral capsid antigen by an immunofluorescence assay. The proportion of subjects with antibodies increased from 15% at 1 yr to 60-65% in the 4-6 yr age group, was stationary at the age of 7-14, and then increased further progressively with age, to reach about 95% in late adulthood. Among preschool children a significantly higher prevalence rate was demonstrated in the lower socioeconomic stratum, as well as a somewhat higher prevalence rate in children attending day nurseries. The study indicates that infections with EBV have a rather early and widespread occurrence in this country compared with other developed areas.