There is some debate regarding the safety of methimazole (MMI) therapy during pregnancy. It is not known whether MMI therapy in mothers during pregnancy is safe for their children or if it causes alterations in thyroid function and intellectual development during childhood. Twenty-three children, whose mothers were hyperthyroid during pregnancy and treated with MMI 5-20 mg were studied from age 3-11 yr. Thyroid function and liver function tests, urinary iodine, anti-thyroid antibodies, intelligence quotient (IQ), verbal and functional components of Wechsler test were performed on 23 children of thyrotoxic mothers and 30 controls. In all children T3, T4, RT3U and TSH concentrations were normal. Mean T3, T4 and TSH values were 147 ng/dl, 9.7 microg/dl and 1.2 mU/l, respectively. Height, weight, thyroid function, and thyroid antibodies did not differ from controls. None of the children had T4 below 6 microg/dl or TSH>3.0 mU/l. Liver enzymes and serum albumin were normal in both groups. Mann-Whitney test showed no difference in verbal and performance IQ and their components between children of thyrotoxic mothers and controls. Total IQ of cases and controls was 117 +/- 11 and 113 +/- 14, respectively. No deleterious effect occurred in thyroid function and physical and intellectual development of children whose mothers were treated during pregnancy with doses of MMI up to 20 mg daily.