Mutations of the thyrotropin receptor (TSH-R) gene have been reported in some cases of hyperthyroidism. We report a case of a family that had a high incidence of hyperthyroidism (6/13) which strongly suggested hereditary factors. We then analyzed whether the family had mutations of the TSH-R gene. No significant mutations in exon 10 of the TSH-R gene were found in the patient by restriction fragment length polymorphism analysis and polymerase chain reaction direct sequencing, when compared with those with 4 normal subjects and 2 patients with Graves' disease. Unknown mutations in the extracellular region of the receptor or other genes in this family remain to be studied.