SUMMARYTen pregnant women with a history of past or present hyperthyroidism were investigated. Three of these patients had evidence of LATS activity in their serum. Two of these had infants who had LATS activity in their cord blood; a pair of twins, and a singleton. These three infants developed neonatal thyrotoxicosis. Serial samples of blood from these infants indicated a half life LATS disappearance of 4.1, 6.7 and 7.6 days respectively.