As a part of studies concerning clinical application of the measurement of urinary iodide with an iodide-selective ion electrode, we report here the reference values for the iodide content or concentration in morning spot urine specimens from normal subjects of varying ages and studies with patients with thyroid diseases in Japan. The number distribution of the iodide content or concentration in the morning specimens appeared logarithmic normal in adults, children and infants. Normal ranges found in 95 per cent of populations of healthy subjects were 5.3 to 62.0 mumoles/g Creatinine (Cr) for adults, 5.3 to 42 mumoles/g Cr for children, and 1.9 to 56 microM for infants, respectively. Urinary iodide concentrations in breast-fed infants varied widely compared with those in bottle-fed infants. Mean values were 16.4 microM for breast-fed infants and 8.6 microM for bottle-fed infants, and they were not statistically different. Urine samples from the infants with transient hypothyroidism, who had undergone amniofetography showed extraordinarily high iodide concentrations, even though they were measured at 20th, 29th and 30th days after birth. Although urinary iodide excretion in patients with simple goiter was within normal limits, the mean was statistically lower than that in normal controls (p less than 0.001). Because of the simplicity and rapidity of the electrode method, we strongly recommend it for use in examining iodide excretion in patients with various thyroid diseases.