Anti-thyroid antibodies are frequently found in otherwise normal populations (4.5–25.8%); however, there is scanty information about thyroid function status in affected individuals. In this report, the serum concentrations of TSH, T3, T4, rT3 and TBG and the titre of anti-thyroglobulin and anti-microsomal antibodies (haemagglutination technique) were studied in 520 healthy school children (260 boys and 260 girls) aged 6.0–17.9 years. Titres equal or greater than 1:16 of one or both antibodies were detected in 58 boys and in 77 girls (in 33 boys and in 24 girls with, and in 25 boys and 43 girls without, associated abnormalities in the serum concentrations of one or several hormones). The age distribution of thyroid antibodies followed a trimodal pattern with peaks at 7, 11 and 16–17 years in both sexes. The most striking finding was an abnormally elevated T3 concentration in 22 boys and 5 girls with positive antibodies, with no symptoms of thyroid dysfunction and with no clear relationship with simultaneous abnormalities in TSH, T4 or rT3; however, in 5 boys the TBG serum levels were increased. Serum from these patients was incubated with [125I]T3 before free radioactivity was precipitated with dextran-coated charcoal and the aliquots were analyzed by paper electrophoresis. Serum samples with high T3 levels bound significantly more radioactivity than normal or T3-free serum (P < 0.001) and an abnormal peak of radioactivity was present in the gamma globulin fraction, in the former but not in the latter two types of sera. The presence of high serum T3 levels in the absence of clinical symptoms of hyperthyroidism was probably due to sequestration of T3 by the anti-thyroglobulin antibody, which may have cross-reactivity with T3 and T4, as has previously been demonstrated both in animals and humans.