Transient neonatal hypothyroidism induced by transplacental transfer of thyrotropin receptor-blocking antibodies is rare, but should be diagnosed early because its course, treatment, and prognosis are different from the other forms of congenital hypothyroidism. Transient neonatal hypothyroidism should be suspected in infants with a history of maternal autoimmune thyroid disease. We describe two siblings whose mother has been treated for idiopathic primary nongoitrous hypothyroidism since adolescence. High levels of thyrotropin receptor-blocking antibodies were first detected in maternal serum at the time of the first child's birth. At the time of the second child's birth antithyroid peroxidase antibodies were found in addition to the thyrotropin receptor-blocking antibodies. Both children were clinically healthy newborns without evidence of congenital malformations. Thyroid suppression, reflected by high levels of TSH during neonatal screening, was transient in both infants. Hormonal substitution was only necessary in one child for a period of 4 months. When tested at the age of 6-7 months, maternal antibodies had completely disappeared from the infants' sera. At ages 7 and 4 years respectively the 2 children do not require treatment and show normal growth and neurodevelopmental skills. In the north-eastern part of Switzerland congenital hypothyroidism has an incidence of one in 3300 live-born infants, the most frequent form being permanent congenital hypothyroidism (1:4500). In this population, analyzed over a period of 16 years, the two cases reported represent the only observations of transient neonatal hypothyroidism due to thyrotropin receptor-blocking antibodies; the incidence can thus be estimated to be approximately 1:310,000 live newborns. In this rare condition, early recognition is pertinent in order to avoid unnecessary treatment and parental anxiety.