The detection of nodular lesions in the periphery of the lung by roentgen examination is now a common occurrence. Lesions as small as 3 mm. in diameter can be found when in a favorable position, and primary bronchogenic carcinomas, as well as metastases, as small as this have been reported. The major problem involved in the consideration of such lesions is the differential diagnosis as to the nature of the process producing what is commonly designated as a “coin” lesion. It has been estimated by some authors (2, 6, 12, 16) that there may be as many as thirty different causes responsible for the production of such a shadow. In 1948, one of us (L. G. R.) first observed a curious irregularity or, as it was then called, an umbilication, on the margin of a shadow in a lateral roentgenogram of the lung, in a case later proved to be undifferentiated carcinoma. The repeated observation of a similar finding in planigraphic studies of spheroidal nodules in the lung, which later proved to be malignant tumors, has led to a more extensive study and to a preliminary report of this finding as indicative of malignancy (14). With this particular sign in mind, we have reviewed all our cases of pulmonary disease found during the past five years in which planigraphic studies were made. Our object was to determine the value of such studies in the differential diagnosis of the peripheral lung nodule and to evaluate the significance of this “notch” sign in the diagnosis of pulmonary carcinoma. There were available to us some 550 cases in which lesions in the lung were suspected and more or less adequate planigraphic studies had been undertaken. Many of the studies proved to be insignificant; others were made for clear-cut tuberculosis and for similar reasons. We were able, however, to find some 132 cases in which planigrams were made to determine more clearly the nature of a peripheral nodule. In order to define the cases considered, we would clarify the term “peripheral nodule.” As used by us, it refers to what is commonly miscalled a “coin” shadow but actually represents a spheroidal nodule in the lung. Cases have been selected in which a density can be seen in the lung almost completely surrounded by air, so that it is clearly not a part of the mediastinal structures or the major bronchi and has a general, although not necessarily clear-cut, spheroidal form. The size has varied from 1 to 10 cm. in diameter. In 50 of the 132 cases the diagnosis was verified histologically. Of the proved cases, 24 were primary carcinomas of the lung and 8 metastases from various sources. There were 11 granulomas and 4 hamartomas. In 1 case the histologic evidence did not establish the diagnosis; in 2, exceedingly small lesions were actually not found at autopsy although the examination was not intensive enough to exclude them.