Plasma thyroxine (T4) and thyrotrophin (TSH) were estimated in 34 children identified by neonatal hypothyroid screening and subsequently found to have ectopic thyroid tissue on isotope scan. Before treatment plasma T4 ranged from 8-143 nmol/l and TSH from 39-1,230 mU/l. After one week off treatment during their second year, repeat T4 in 26 of these cases showed a significant correlation with the pre-treatment values (r = 0.57). However, only 3 of the 5 children with pre-treatment T4 levels over 100 nmol/l at diagnosis had normal T4 values when retested. Similarly, when 10 children with pre-treatment T4 values over 65 nmol/l were retested off treatment at the ages of 5.8-8.2 years, only 4 had plasma T4 levels in the normal range. These results illustrate the wide range of thyroid function which can occur in children with ectopic thyroid tissue and indicate that some continue to have near-normal thyroid function for considerable periods. However, pre-treatment T4 results do not allow accurate identification of these latter cases.