Sera from 52 patients with Turner's syndrome were analyzed for thyroxine and TSH concentrations, resin T3 uptake, and thyroid antibodies. Thyroid antibody titers were compared to those previously obtained in 53 women with long-standing infertility. Thyroid microsomal and/or thyroglobulin antibody titers were elevated to a level diagnostic of Hashimoto's disease (greater than 1:400) in 25 (48%) patients with Turner's syndrome. Microsomal antibody levels only were elevated in five (9%) patients with infertility. The mean microsomal antibody titers in the patients with Turner's syndrome (1:25, 167 +/- 31,531) were significantly higher than in the infertile patients (1:2560 +/- 2149). The incidence of Hashimoto's disease did not differ significantly among the various karyotypes. The entire clinical spectrum of Hashimoto's thyroiditis was present in Turner's syndrome. Three patients had overt hypothyroidism necessitating L-thyroxine, four had compensated hypothyroidism (serum TSH concentration greater than 10 microU/L and normal serum thyroxine concentrations), while the remaining 18 had normal thyroid function. No relationship was demonstrated between Hashimoto's disease or family history of thyroid disease and karyotype.