Roentgenographic and histologic examinations were carried out on excised specimens of 100 cases of malignant thyroid neoplasms and of 98 cases of benign nodules. Industrial film and low voltage were used to obtain the maximum detail. Microscopic sections stained with hematoxylin and eosin were examined in all the cases and the selected 24 sections were stained by von Kossa's method. Calcium deposits were grossly classified into psammomatous and coarse types. The former was seen in 59 of 100 malignant tumors and in only 2 of 98 benign nodules, whereas the latter was observed equally in both the malignant and benign nodules at the rate of 42%. Roentgenographic-histologic study revealed that most of the psammomatous calcifications were produced by true psammoma bodies, but there were a few exceptions, which were minute calcific deposits in the fibrous stroma and, in the cases of the medullary carcinoma, were calcifications in the amyloid. Several modes of development and distribution of psammoma bodies were found and each example was shown. Coarse calcifications were present in areas of fibrosis and showed various configurations. In only 2 adenomas, calcific deposits were seen along the sinusoidal vascular structures within the tumor parenchyma and fleece-like shadows were obtained on x-ray films. Other types of coarse calcification have no relationship to histologic diagnosis.