Autoantibodies to T3 (T3Ab) were detected by immunoprecipitation in a 18-year-old female patient affected by Graves' disease. The presence of these antibodies was constantly confirmed during a 7-year follow-up period, independently of therapy and functional thyroid status. Antithyroid microsomal antibodies (McAb) and TSH binding-inhibiting antibodies (TBII) were also present. The title of AbT3, McAb and TBII fluctuated, at times, independently of one another. The patient became pregnant; during pregnancy T3Ab concentration fell and went up again 4 months after delivery. A normal baby was born. T3Ab were present in the cord blood and declined during the following 2-8 months. The T3Ab of both mother and child belonged to the IgG class. The concentrations and the binding affinities of T3Ab in mother and child were identical. The presence of T3Ab in the child did not affect his thyroid function.