Objective: To evaluate the effect of passive smoking on thyroid function in infants. Design and methods: Cord serum tri-iodiothyronine (T 3 ), free T 3 (fT 3 ), thyroxine (T 4 ), free T 4 (fT 4) , TSH, thyroxine binding globulin (TBG), thyroglobulin (TG) and cord plasma thiocyanate were measured at birth, and serum TG and thiocyanate after 1 year of life, in 18 infants born from parents who did not smoke (group A), in 18 infants with a father who smoked (group B) and in 18 infants with parents both being smokers (group C). Results: No significant differences were observed in cord serum concentrations of T 3 , fT 3 , T 4 , fT 4 , TSH and TBG among the three groups. Median (range) TG concentrations (ng/ml) were 30. Conclusions: TG and thiocyanate concentrations at birth and at 1 year of age in infants of smoking parents are greater than in infants with non-smoking parents. These results indicate that the change in thyroid function as evaluated by serum TG concentrations observed at birth can persist at least for 1 year if the exposure to passive smoking from both parents is continued. Increased TG concentrations may be due to a direct effect of thiocyanate on the thyroid gland.