In order to evaluate whether or not iodine intake in Japanese is variable among different aged subjects and also whether this variation is concordant with the age distribution in patients with chronic thyroiditis, we tried to examine the urinary iodine excretion of euthyroid people in various decades of age. One hundred and twenty outpatients without thyroid disorders, aged 22 to 77 y.o., were selected at random and were divided into 6 groups according to age. Mean total urinary iodine excretion (UIT) and concentration (UIC) were 336.1 micrograms/day and 31.4 micrograms/dl/cr, respectively, and they were in good correlation (r = +0.82, p < 0.001). A straight line on log normal probability was recognized between the ratio of cumulative frequency and values of UIT. UIT and UIC were significantly correlated with serum nonhormonal iodine, respectively (r = +0.21, p < 0.01; r = +0.28, p < 0.01). These indicators in the 3rd decade were lower than those in the 6th decade (p < 0.01, p < 0.05). UIT increased with age up to the 6th decade and then decreased gradually to the 8th decade. Therefore, UIT in all the subjects formed a bell-shaped distribution with a significant peak in the 6th decade (H = 12.1, p < 0.05). Rates of renal iodine clearance (UIC/SNI) in the 6th decade increased significantly more than those in the 4th decade (p < 0.01), and the distribution of those rates in the 6 groups were similar to that of UIT.(ABSTRACT TRUNCATED AT 250 WORDS)