OBJECTIVE The higher frequency of Hashimoto's thyroiditis in Turner's syndrome compared with the general population is well known. We have attempted to establish clearly the more frequent association of thyroiditis with the X-isochromosome, since previous reports of this aspect have included only small numbers of patients. METHODS Retrospective study of patients with Turner's syndrome investigated within the past 12 years. METHODS Sixty-seven cases of Turner's syndrome were reviewed. METHODS Peripheral blood leucocyte karyotype and screening for thyroid disturbances on the basis of clinical examination and laboratory evaluation (anti-thyroglobulin and anti-microsomal antibodies, basal TSH levels and TSH levels after TRH stimulation) were made for each patient. RESULTS A diagnosis of thyroiditis, based on the association of positive antibody titres, elevated TSH and an abnormal thyroid gland on clinical examination, was established in 20.9% (14/67) of cases. A significantly higher frequency of thyroiditis was found among the patients presenting with an X-isochromosome (57.3%, 9/16), compared to patients with other karyotypes (9.8%, 5/51) (P = 0.0001). CONCLUSIONS Our results, obtained by investigation of a larger number of patients with an X-isochromosome karyotype than in previous reports, confirm conclusively that patients with X-isochromosome Turner's syndrome have an increased risk of developing thyroiditis.