Hyperthyroidism during pregnancy may be dangerous to the infant. The major risks are prematurity and neonatal thyrotoxicosis. The latter may be due to placental transfer of thyroid stimulating immunoglobulins from mother to fetus. Of 2 siblings of a previously thyrotoxic mother, the 1st had marked symptoms of neonatal thyrotoxicosis after a pregnancy where no antithyroid treatment was given. The 2nd child had only minimal thyrotoxic symptoms, but almost as high levels of thyroid hormones as the 1st. During the 2nd pregnancy propylthiouracil was given to the mother from 26 wk gestation, because of increased fetal movements and fetal tachycardia. Fetal movements and fetal heart rate were apparently most valuable indicators of thyroid function in the fetus. Intense control is necessary from the beginning of the 2nd trimester.