Transient neonatal hyperthyrotropinemia (TNH) occurs frequently in areas of iodine deficiency. To evaluate the effect of TNH in intellectual function and psychomotor performance, a historical cohohrt study was performed in 9 years old children with documented TNH at birth. 18 children with TNH who had been born in Mahdieh Hospital were studied at age 9 and compared to 19 matcheal children born at the same time, but having normal thyroid function at birth. Global intelligence (IQ) and psychomotor performance were evaluated with Raven and Bender-Gestalt tests, respectively. Total serum T4 and T3 by commercial RIA and TSH by IRMA. Urine was tested for iodine content by digestion method. Height and weight were similar in two groups at birth and at 9 years of age. Thyroid function tests were similar in the two groups except for TSH at birth which was higher in TNH than in control group (23.4 +/- 8.3 vs 3.6 +/- 1.0 mU/L, P < 0.001). Results of T4, T3, resine uptake, and urinary iodine at 9 years of age were not different between two groups. Mean IQ was 98 +/- 11 and 106 +/- 8 in TNH and normal children, respectively (P < 0.01). There was no significant difference between psychomotor performance in the two groups. There was no correlation between TSH at birth and IQ at 9 years of age. The present finding suggests that TNH can adversely affect longterm intellectual development.