A seroepidemiological study was conducted in 329 Swedish children aged 3-12 years, some of whom had upper respiratory tract infections, in order to determine the prevalence of serum IgG and IgM antibodies to Chlamydia trachomatis in this age group. Serum antibodies were detected by microimmunofluorescence assay. IgG antibodies were found in 28 (8.5%) of the children; titers were greater than or equal to 1:16 in 15 children (4.6%). Their prevalence in boys (9.4%) and girls (7.6%) was not significantly different. IgM antibodies were found in only six children. Their peak prevalence occurred in boys at the age of 8, and in girls at the age of 12. The serological findings are discussed in the light of current knowledge about chlamydial infections in children and the specificity of chlamydial antibody tests.