We have been developing an X-ray fluorescence system designed to determine iodine distribution and content in the thyroid. The X-ray fluorescence unit is composed of a 80 KV X-ray excitation beam and a Si(Li) semi-conductor detector. The angle between the X-ray beam and the axis of the detector is 24 degree. Two single channel analyzers (SCA) are used, the first one corresponding to the K alpha peak and the other to the background. The SCA signals are digitised so that the two corresponding images may be processed using a computer. After subtracting the background, an image is obtained that represents the iodine distribution alone, and the iodine content of the whole thyroid or a part of it can be determined with a reproducibility of +/- 5%. A good correlation (r=0.98) between iodine chemical analysis and iodine fluorescence analysis was obtained for 13 patients.