Recently, a few studies have shown the safety of methimazole (MMI) therapy of thyrotoxic lactating mothers on thyroid function of their infants. However, it is not known whether the effect of moderately high doses of MMI therapy on lactating mothers can be dangerous for breast-fed infants. Eighty-eight thyrotoxic lactating mothers and their infants were studied. 46 received 20 mg MMI and 42 were given 30 mg MMI during the first month, 10 mg for the second and 5-10 mg for additional 10 months of therapy. Serum T4, T3 and TSH concentrations and in hyperthyroid MMI treated mothers and their RT3U were measured in hyperthyroid MMI treated mothers and their infants, before and at 1, 2, 6, and 12 months after initiation of therapy. Serum MMI was measured in the infants of thyrotoxic mothers taking 20-30 mg MMI. Mean+/-SD of free T4 index (FT4I) in thyrotoxic mothers treated with 20 and 30 mg MMI for one month decreased from 20.1+/-4.2 to 9.7+/-1.5 (p<0.001) and from 20.6+/-4.8 to 8.6+/-3.0 (p<0.001), respectively. Values for free T3 index (FT3I) decreased from 587+/-53 to 180+/-39 (p<0.001) and from 610+/-49 to 151+/-31 (p<0.001) in those treated with 20 and 30 mg MMI, respectively. By the end of one month 5 had elevated FT4I or FT3I or both and 12 had elevated TSH. The dose of MMI was adjusted and thyroid function remained normal up to 12 months of MMI therapy in thyrotoxic lactating mothers. Serum T4, T3 and TSH concentrations of breast-fed infants were normal before and up to 12 months of MMI therapy of their breast-feeding mothers. The lowest T4 and T3 and the highest TSH values were 101 nmol/l, 1.8 nmol/l and 4.1 mU/l, respectively. Serum MMI levels were <0.03 in 7 and 0.03, 0.034 and 0.035 microg/ml in the other 3 infants. We conclude that the treatment of hyperthyroid lactating mothers with doses of 20-30 mg MMI day does not cause deleterious effects on thyroid function of their breast-fed infants.