We report the intra-uterine and postnatal thyroid status of a newborn, whose mother, affected with Hashimoto's thyroiditis superimposed on a previous Graves' disease, again became hyperthyroid during the third trimester of pregnancy. The mother had very high levels of anti-thyroid auto-antibodies, including TSH receptor auto-antibodies (TRAb) measured as TSH-binding inhibiting auto-antibodies (TBIAb). In order to exclude fetal thyroid dysfunction due to passive transplacental transfer of TRAb, fetal blood samples were obtained by cordocentesis at 21, 27 and 32 weeks of gestation. A transplacental transfer of TRAb was already seen at 21 weeks, but no alteration of fetal thyroid function was present at that time. In the following weeks, a rise in TRAb and circulating thyroid hormones was observed both in the fetus and mother, accompanied by overt hyperthyroidism in the mother and by growth retardation in the fetus. At birth, TRAb were shown to have stimulating activity both in the newborn and mother. This report documents the early transplacental passage of thyroid auto-antibodies and underlines the importance of close follow-up of pregnant women with auto-immune thyroid disorders.