We compared fetal and maternal serum indexes of thyroid status at delivery in 70 patients with Graves' disease who required therapy with thionamides (such as propylthiouracil) during pregnancy. Forty-three mothers required thionamides until delivery (Group 1), whereas the drugs were discontinued during pregnancy after remission in 27 mothers (Group 2). Maternal free thyroxine levels were closely correlated with cord levels in both groups, being essentially identical in Group 2 but slightly lower in fetuses than in mothers in Group 1. Normal maternal free thyroxine levels did not preclude fetal hypothyroidism. The mothers and fetuses in Group 1 had a significantly higher incidence of antibodies that inhibit thyrotropin binding than did those of Group 2. However, a significant correlation between maternal levels of these antibodies and cord levels of free thyroxine or triiodothyronine was found only in Group 2, in which some maternal and cord thyroxine levels were in the thyrotoxic range at delivery, presumably because therapy was discontinued. These findings indicate that high free thyroxine levels and the presence of antibodies that inhibit binding of thyrotropin are useful indexes of the fetal need for antithyroid treatment, and that the thionamide dosage that maintains maternal free thyroxine levels in a mildly thyrotoxic range seems appropriate for maintaining euthyroid status in the fetus.