Transient neonatal hypothyroidism was found in a boy whose mother was treated for hypothyroidism due to Hashimoto's thyroiditis. During the neonatal period the infant had antithyroid microsomal and antithyroglobulin antibodies and immunoglobulins inhibiting cyclic AMP production by thyroid cells in vitro. After one year of treatment, all antibodies disappeared. Thyroid scintiscan and fixation in the neonatal period was negative and became positive 2 months after stopping treatment with normal fixation and cervical thyroid picture. The mother's serum contained the same antibodies: they crossed the placental barrier and were responsible for neonatal pathological manifestations.