Untreated hyperthyroidism in pregnant women is associated with a high incidence of maternal and fetal complications. Thus, its treatment is mandatory, ideally using PTU because it has lesser transplacental passage. From 1987 and 1991 we have attended 19 hyperthyroid pregnant women. Of these, 18 had diffuse and 1 nodular goiter and in 10, thyrotoxicosis preceded pregnancy. PTU was used in 17 women (7 received it along the whole pregnancy), five had to be operated due to poor response, one received propranolol and one patient was not treated due to lack of attendance. Cesarean section was performed in 12 women, 5 had vaginal delivery, one had a miscarriage at the 20th week of pregnancy due to a neurological malformation and one patient was lost from control before delivery. The newborn of the untreated woman had a neonatal thyrotoxicosis and the resting 16 did not show evidence of thyroid disfunction. Newborns from mothers receiving PTU until delivery had significantly lower rT3 levels and non significant changes in T4 and T3. At the end of the observation period, 8 patients were euthyroid, 3 hypothyroid (2 after 131-l and 1 after surgery), 4 continued on PTU and 4 were lost from control. It is concluded that the outcome of pregnancy may be uneventful in hyperthyroid women provided that there is a close and adequate follow up.