Thyroid antibodies were determined by three different techniques in the sera of 125 children and adolescents with thyroid disorders and in the sera of 53 short, normal children without thyroid dysfunction. The incidence of antithyroglobulin antibodies in patients with thyroiditis was highest when measured by radioimmunoassay (85%), less than when measured by hemagglutination (24%), and least by antimicrosomal antibodies (7%). No patient who had initially negative serum for RATA subsequently had positive tests during follow-up of five to 24 months, whereas eight of 31 patients with initially negative serum for ATA later developed positive tests. Treatment appeared to have a suppressive effect on RATA, but not on ATA titers, in hypothroid patients with clinical thyroiditis. The incidence of hypothyroidism in the patients with clinical thyroiditis on initial presentation was significant (37%) and suggests that identification of children and adolescents with thyroiditis is important to ensure adequate medical follow-up.