Constitutively activating germline mutations in the TSH receptor (TSHR) gene have been identified as a cause of autosomal dominant nonautoimmune hyperthyroidism and sporadic congenital hyperthyroidism. We report a 10-yr-old boy and his 31-yr-old mother, both presenting with a history of recurring toxic thyroid hyperplasia and no evidence for autoimmune thyroid disease. In the boy, onset of hyperthyroidism and goiter was neonatal. In the mother, onset of thyroid disease dates back to early childhood. There was no history of thyroid disease in the rest of the family. Screening for germline mutations in exon 10 of the TSHR was performed by direct sequencing of genomic DNA extracted from peripheral blood leukocytes of both patients. In the boy and his mother, an identical heterozygous TSHR mutation was identified, exchanging leucine for phenylalanine at residue 629 of the TSHR (TTG-->TTT). Transient expression of the mutated TSHR construct in COS-7 cells confirmed the constitutive activity of the new TSHR germline mutation. This is the second family displaying congenital manifestation of hyperthyroidism in familial nonautoimmune hyperthyroidism.