Objective: Recent studies have shown normal thyroid function in infants whose mothers receive methimazole (MMI) during breast-feeding. This study evaluates the long-term effect of MMI on thyroid function and intellectual development of such children. Design and Methods: Eighty-two children aged between 48 and 86 months were studied. Forty-two children had been breast-fed while their thyrotoxic lactating mothers received daily doses of MMI 20-30 mg in the first, 10 mg in the second and 5-10 mg for additional 10 months of therapy. Thyroid function of infants remained normal during the one year of MMI therapy of their mothers. Forty other infants served as controls. Serum T-4, T-3, and TSH concentrations, urinary iodine, thyroid antibodies, intelligence quotient (IQ), verbal and functional (performance) components (Wechsler and Goodenough tests) were measured in all children of the two groups. Results: Height, weight, serum T-4, T-3, TSH and antithyroid antibody titers were not different between children in the two groups. The mean IQ was 107 +/- 17 vs 106 +/- 16 (Goodenough test) and 103 +/- 10 vs 103 +/- 16 (Wechsler test) for children of thyrotoxic mothers and control children, respectively. There was no difference in verbal and functional IQ and their components between children of thyrotoxic MMI treated mothers and control children. Conclusion: Thyroid function and physical and intellectual development of breast-fed infants whose thyrotoxic lactating mothers were treated with 20-30 mg doses of MMI daily are normal at age 48 to 86 months.