Iodized oil administration has not been effective in restoration of euthyroidism in adolescents and adults, and only partially effective in children aged 4 to 14 years with endemic myxedematous cretinism. Therefore, we studied the effect of iodized oil injection in euthyroid and hypothyroid school girls in an endemic region. Clinical examination and assessment of serum thyroid hormones and TSH were performed before and at 4, 7 and 12 months after intramuscular injection of 480 mg iodine in 54 school girls. Four months after iodized oil injection, T4 and T3 increased from 58 +/- 24 and 2.6 +/- 0.6 to 127 +/- 28 and 3.1 +/- 0.5 nmol/l, respectively. Serum TSH was above 5 mU/l in 45 girls before and in 4 girls four months after injection. None had increased TSH at 7 and 12 months of study. Older girls had lower T4 and higher TSH than younger ones, however, all responded well to iodized oil administration. Serum T3 was above the normal limit of 3.1 nmol/l in 16 subjects. There was no clinical evidence of hyperthyroidism. Mean serum T3 returned to pre-treatment value by 7 and 12 months. TSH response to TRH was subnormal in 5 girls who had increase in either T3 or T4 or both at the end of one year. Injection of iodized oil reverses thyroid function in children and adolescents with hypothyroidism, when the severity of iodine deficiency is not enough to cause thyroid cell atrophy. Transient elevation in serum T3 and/or T4 occurs in many and may last in some at least one year following injection.