We describe Japanese siblings with resistance to thyrotropin (TSH) who are compound heterozygotes for two novel mutations in the TSH receptor gene. The affected siblings had increased serum TSH, normal serum thyroid hormones, and normal positioned but slightly hypoplastic thyroid glands. The mutated paternal allele has the substitution of His (CAC) in place of Arg (CGC) at codon 450 (R450H) of the TSH receptor. The mutated maternal allele has the substitution of Ser (AGT) in place of Gly (GGT) at codon 498 (G498S) of the TSH receptor. COS-7 cells transfected with the R450H mutant exhibited a slightly decreased TSH binding and a slightly decreased cyclic adenosine monophosphate (cAMP) response to TSH, whereas cells transfected with the G498S mutant exhibited a markedly decreased TSH binding and a markedly decreased cAMP response to TSH. Flow immunocytofluorometry analysis demonstrated that the G498S mutant resulted in extremely low expression at the cell surface as compared with the wild type receptor and the R450H mutant, in spite of a normal intracellular synthesis. The present cases are the first Japanese patients with TSH resistance in whom mutations in the TSH receptor gene have been identified. These novel mutations may contribute to understanding of the struc-ture-function relationship of the TSH receptor.