Thyroid dysfunction, especially hypothyroidism caused by Hashimoto's thyroiditis is more frequently observed in girls with Turner's syndrome (TS). The aim of the present study was to evaluate prevalence, etiology, karyotype distribution and age at onset of thyroid pathology in girls with TS. Data recorded in 84 girls with TS attending our clinic were analyzed. The mean age +/- standard deviation [SD] at their initial evaluation was 10.3 +/- 3.7 years (range, 0.5 to 19 years) and the mean period of observation was 8.4 +/- 4.4 years. The thyroid function had been evaluated at least once per year in all patients and thyroid autoantibodies (ATA) were available in 51 (60.7%). Hypothyroidism was detected in 24% of the studied subjects and hyperthyroidism in 2.5%. Elevated values of thyroid autoantibodies were detected in 42% of girls with TS, whose ATA had been determined, and 65% had hypothyroidism. Thyroid dysfunction was first noted after the age of 8 years with no difference in the distribution of new cases at the different ages or pubertal stages. There was no difference in the incidence of thyroid dysfunction related to the type of karyotype abnormality. Thyroid dysfunction is more frequently encountered in girls with TS (hypothyroidism: 24% in the total group and 65% in those with positive ATA, hyperthyroidism: 2.5%). Thyroid dysfunction was observed after the age of 8 years with no difference in the occurrence of new cases in the various age groups thereafter. Hence, thyroid function should be evaluated yearly in girls with TS past the age of 8 years and more frequently in those with positive thyroid autoantibodies.