A case of fetal thyrotoxicosis in utero in a boy with familial predisposition to thyrotoxicosis is reported. At 26 weeks of gestation fetal hyperkinesia and fetal tachycardia developed. This was preceded by a significant pathologic increase in long-acting thyroid stimulator (LATS) in the mother. The fetal hyperkinesia and tachycardia were considered to be signs of fetal thyrotoxicosis, possibly induced by placentally transferred LATS. The fetal thyrotoxicosis responded well to propylthiouracil given to the mother. After birth the boy developed slight signs of neonatal thyrotoxicosis although his serum thyroxine values increased analogous to those of his elder sister, who had presented classical neonatal thyrotoxicosis.