Congenital hyperthyroidism is a very rare disease. But, for each affected child it has to be considered as a serious condition because of the negative impact of hyperthyroidism on fetal and postnatal development. If the manifestation occurs during fetal life tachycardia, cardiac arrhythmia, growth retardation and, most significant, prematurity are the consequences. Postnatal signs of hyperthyroidism are irritability, tachycardia, hypertension, poor weight gain and thyroid enlargement. Even cardiac failure may occur if hyperthyroidism is severe and treatment not adequate which explains the high early mortality rate of 16%. The main complication of persistent hyperthyroidism in the neonatal period and during infancy is craniosynostosis. Severe developmental delay or even mental retardation can be the consequence of inadequate high T4-levels during fetal and neonatal life. Congenital hyperthyroidism was first recognized in infants born to mothers with Graves' disease. The description of transplacental passage of the maternal thyroid stimulating antibodies elucidated the molecular mechanism in this major group of patients with "autoimmune congenital hyperthyroidism". In contrast to this transient, self-limited character of "autoimmune congenital hyperthyroidism", due to the clearance of maternal antibodies from the infant's circulation, some cases of persistent congenital hyperthyroidism without signs of thyroid autoimmunity have been recognized. Activating mutations in the thyroid-stimulating hormone receptor were described recently as the underlying molecular pathogenesis in this group of "non-immune congenital hyperthyroidism". Therefore the possibility of a molecular differential diagnosis of both groups of congenital hyperthyroidism now exists and opens the opportunity of optimal treatment for each patient.