TSH antireceptor antibodies (TRAb) and antithyroid drugs cross the fetoplacental barrier and produce transient neonatal hyperthyroidism or hipothyroidism independent from thyroid maternal status. We present our experience in the follow up of 61 newborn to mothers with Graves disease (GD). Samples were obtained from cord blood, at 72 hours and 7 days of life. measuring TSH, T4, T3,AFM, and TRAb. We found: A) 4 patients (6.7%) showed clinical and biochemical signs of hyper-thyroidism at birth with advanced bone age (2 from mothers treated with MM1 and 2 from mothers without treatment). All these patients presented TSH levels in cord blood <0.15 uUl/ml and T4 between 20 and > 30 ug%. In 3 of them TRAb were 59% and 99%. They improved with MM1 treatment at 1 mg/kg/day for 30 to 60 days. The follow up showed a normal neurological development. B) 5 patients (8.2%) presented hyperthyrotropinemia (TSH range 24-50 uUl/ml in cord blood) from mothers treated at 5 - 20 mg/day MM1 in the last trimester of gestation. C) 2 patients(3.3%) presented hypothyroidism: TSH=75 uUl/ml in both, T4=3.6 in one and 4.8 ug% in the other from mothers treated at 40-45 mg/day MM1 in the last trimester of pregnancy. No patients from group B or C had clinical symptomatology. Their bone age was adequate for gestational age. Biochemical values normalized since day 8 th. Hyperthyrotropinemia was found also in a 21 days newborn (TSH= 30 uUl/ml) whose mother followed receiving 5 mg MM1 during lactation; TSH level normalized at day 7 th after MM1 was stopped. Conclusions: 1) High incidence of neonatal hyperthyroidism (6.7%) could be cansed for a systematic sampling in newborn to mothers with autoimmune thyroid disease. 2) Usefulness of TRAb dosage for the diagnosis and follow up of these children. 3) Maternal MM1 doses above 20 mg/day could produce hypothyroidism and only hyperthyrotropinemia below this value. 4) 5 mg MM1 would be enough to provoke hyperthyrotropinemia during lactation. 5) Importance of systematic monitorized follow up of newborn to mothers with autoimmune thyroid disease for an early detection of this patology.