Analysis by x-ray fluorescence allows in vitro determination of iodine content of the thyroid gland and a mapping of the regional distribution of iodine in the gland. The picture produced is similar to that of the conventional radioisotope thyroid scan. In 5 normal subjects and 70 patients with thyroid disease, the thyroid concentration of iodine varied between undetectable and 28 mg. With the exception of hypothyroid patients, who showed low thyroid levels of iodine, all patients showed iodine values overlapping the normal range. The fluorescent scan gave results similar to those of the isotope scan in most cases. Exceptions were noted in some hypothyroid patients, patients with flooded iodine pool, and patients receiving suppressive doses of exogenous thyroid hormone. Small cold nodules were best detected by the radionuclide scintigram. The value of the fluorescent scan is in its low radiation dose and in the possibility it affords of studying patients whose thyroid glands have reduced uptake of the radioactive tracer.