DURING the course of radioiodine therapy, we have been interested in measuring the amount of radioiodine retention in functioning thyroid carcinomas for two main reasons: 1) to find how the degree of retention changes from dose to dose, and whether this measurement offers a criterion as to the progress of treatment, and 2) to determine the speed of radioiodine turnover in these tumors, since this is relevant to the amount of radiation delivered by each dose. For both of these purposes, a rapid and robust method is needed for measuring the radioiodine content of the tumor or of its metastases daily after each dose, and under comparable conditions from year to year during treatment. Several procedures might be used for this purpose, and we have obtained serial measurements extending over two and a half years in a group of patients with functioning tumors by the following method: METHODS The patient lies on a couch over which a counter is moved horizontally, the counting rate being determined at points along the length of the body (1). The counter is so shielded as to respond to the radioiodine in the whole width of the body but in only a short section of its length opposite to the counter. In this way a “profile” curve may be drawn, indicating the isotope distribution along the length of the body; and peaks on this curve will correspond to sites of radioiodine concentration. The organs or positions with which these peaks correspond may then be identified by conventional mapping methods. The counting rate at peaks opposite to tumor deposits may be related to the percentage of the dose concentrated in the tumor tissue.