An infant with neonatal hyperthyroidism was born to an euthyroid mother previously treated for hyperthyroidism with sodium iodide I 131. Measurable levels of long-acting thyroid stimulator (LATS) were found in mother and infant. A review of previous cases of neonatal hyperthyroidism in which LATS determinations have been performed indicates the importance of LATS in the pathogenesis of this condition. Infants born of mothers with hyperthyroidism or with a history of treated hyperthyroidism should be evaluated in the neonatal period for hyperthyroidism, particularly if measurable levels of LATS have been found in the mother's serum late in pregnancy. Early treatment of neonatal thyrotoxicosis with propylthiouracil is recommended for severely affected cases.