It is well established that an adequate iodine intake prevents iodine deficiency disorders. Prophylaxis through iodized salt is able to correct urinary iodine deficiency and to prevent goiter endemia, but scanty data are available about its effect on decreasing the thyroid size in goitrous children born before prophylaxis. The prevalence of goiter was evaluated by ultrasound in the school-children population of an area of Eastern Tuscany (Tiberina Valley) characterized by moderate iodine deficiency in 1985. At present, after the implementation of voluntary iodized salt consumption, iodine urinary excretion was borderline sufficient (median, 98 micrograms/L). Goiter prevalence was higher at ultrasound (17%) than by palpation (10%). The median thyroid volume ranged from 3.1 mL in 7-yr-old children to 9.2 mL in 14-yr-old children. In the 7-10 yr age class (i.e. in children born after iodine prophylaxis), no statistical difference in thyroid volume was found with respect to controls. In older children (11-14 yr) born before the institution of iodine prophylaxis, the median thyroid volume was significantly higher than that in age-matched controls. Moreover, in this cluster of subjects, the median thyroid volume in nongoitrous children was higher than that in controls. In conclusion, the data of the present study indicate that the iodized salt prophylaxis is able to prevent the development of goiter in children born after the implementation of iodized salt consumption and to further control thyroid enlargement in older children, but is less effective (or rapid) in reducing goiter size in children exposed to iodine deficiency in the first years of life.