Congenital hyperthyroidism is a rare, transient disease usually caused by transmission of thyrotropin receptor autoantibodies from the mother with Graves' disease to her child. We report a German women and her two sons who had congenital, but persistent hyperthyroidism without any signs of autoimmunity. Direct sequencing of the polymerase chain reaction–amplified exon 10 of the thyrotropin receptor genomic DNA revealed in the mother and both sons a transition of GCC to GTC, resulting in an exchange of alanine 623 to valine. This germline mutation in a highly conserved region of the thyrotropin receptor resulted in a constitutive activation of the cyclic adenosine monophosphate–generating cascade with resulting hyperthyroidism. Analysis of the family for a corresponding BstXI restriction–site polymorphism revealed heterozygosity for this mutation in the affected family members, but not in the father or other relatives. We conclude that whenever congenital hyperthyroidism is persistent and parameters of autoimmunity are absent, a constitutively active thyrotropin receptor mutation should be considered. Treatment appears to require aggressive means such as total thyroidectomy or ablation by ¹³¹iodine because two subtotal thyroidectomies in the mother were insufficient to control the disease. (J Pediatr 1997;131:899-904)