The prevalence of thyroid autoantibodies, i.e. thyroglobulin antibodies and antibodies to thyroid peroxidase, was analyzed in 89 girls, aged 3-16 years (mean age 10 years), with Turner's syndrome. The analyses were performed before the start of growth-promoting treatment and during a follow-up period of 1-5 years. The patients were divided into four groups according to karyotype as follows: group 1, 45, X (n = 63); group 2 with structural abnormalities of the X chromosome (n = 10); group 3 with mosaicism but no structural abnormalities of the X chromosome (n = 10); and group 4, with isochromosome X of the long arm (n = 12): 199 healthy girls aged 12 years, served as controls. Thyroid autoantibodies were demonstrated in 46 of 89 (52%) patients with Turner's syndrome compared with 34 of 199 (17%) age-matched control girls (p < 0.001), thus confirming the relationship between thyroid abnormalities and Turner's syndrome. There was also an increase in the prevalence of thyroid antibodies with age. Simultaneous presence of both autoantibodies was significantly more frequent in group 1 (45, X) and group 4 (isochromosome X of the long arm) than in group 3 (mosaicism) (p = 0.04 and p < 0.002, respectively) and significantly more frequent in group 4 than in group 1 (p < 0.05). During 12-60 months of growth-promoting treatment, no increase in the prevalence of thyroid antibodies was observed. The findings demonstrate the importance of continuous monitoring of thyroid function in girls with Turner's syndrome.