Objective: In a previous study in 1983, goiter was found to be endemic in Shahriar, Iran. Iodized salt has been distributed in the region for the past 12 years, and the present study was performed to examine the effect of iodide supplementation on indicators of iodine deficiency (IDD) and thyroid antibodies. Design & Methods: A total of 3164 people, 58% women and 42% men, were selected by random sampling from the Shahriar area. Goiter was staged according to World Health Organization guidelines. Urinary iodine was measured by a digestion method, and thyroid hormone measurements were done by radioimmunoassay. The results were compared with those of 1983. Results: Goiter prevalence before and after iodine supplementation was 50 and 40% in men, 70 and 51% in women, and 60 and 47% in the whole community, respectively (p < 0.001). A decrease in the prevalence of goiter was observed especially in younger individuals. The mean urinary iodine excretion was 7.6 and 18.5 mug/dL, before and after iodine supplementation. In 1983, the urinary iodine in 47.5% of the population studied was between 2 to 5 mug/dL, while in 1995, 65% of the population studied had urinary iodine between 10 to 25 mug/dL, 12 years after iodine supplementation. Mean serum T4, T3, and thyroid-stimulating hormone (TSH) were normal before and after intervention. There was no significant change in occurrence of positive antibodies, or of hypo-and hyperthyroidism, following iodine supplementation. Conclusion: The result of this study shows that the use of iodized salt causes an increase in excreted urinary iodine and a decrease in the prevalence of thyroid goiter, especially in younger age groups,. Consumption of iodized salt with 40 parts per million (ppm) iodine has not caused increased prevalence of thyroid dysfunction in this area.