IN 1860 Bristowe, in an address before the London Pathological Society, called attention to the fact that metastatic lesions are often found in the lungs, secondary to malignant conditions elsewhere. He stated that these lesions are capable of secondary degeneration and infection, thereby simulating many pulmonary diseases. Clinical diagnosis of metastatic lesions of the lungs was extremely infrequent, however, prior to the discovery of the roentgen rays. This was due partly to the fact that such lesions were not sought for, but chiefly to the infrequency of characteristic physical signs. Certain it is that the extended use of roentgen rays in the diagnosis of pulmonary conditions has changed the previous conception that secondary pulmonary tumors are not common. The first reported case in which roentgen rays were shown to be of value in the demonstration of a pulmonary metastatic lesion was that of Leo, reported in 1898. A boy, aged 10 years, who had been operated on for a sarcoma of the knee, exhibited the clinical manifestations of an organic pulmonary disease. Because of the patient's severe dyspnea, and the long exposures then necessary, roentgenography was not successful, but roentgenoscopy demonstrated the lesions. This method of approach proved to be far superior to ordinary clinical methods. At necropsy it was found that the lesions had been accurately localized by fluoroscopy; it had not been possible to elicit the physical signs by auscultation over the thorax. Following this observation there was a long silent period. There are no available reports of the roentgenologic demonstration of pulmonary metastasis until 1909, when Arnsperger described the characteristics and the development of the lesions as seen by roentgen rays. Following this there was renewed interest in this study, and many reports have been written. The comparative paucity of reported cases may be explained by the fact that they are deemed too common to be of interest, or that, since the condition is terminal, the primary growth is not sought, and as a consequence its presence is never proved. In a study of 71 cases, Moore and Carman found that the primary tumors might be found in the breast, thyroid gland, kidney, suprarenal gland, lung, larynx, esophagus, uterus, prostate gland, testis, sigmoid, in the soft tissues of the hand, arm, shoulder, leg or thigh, in the neck, face or hard palate, and in abdominal or pelvic masses, the nature of which could not be determined. In 20 cases, 28 per cent of the total number, the primary tumor was in the breast. This agrees with Warfield's observation that secondary carcinoma of the lung occurs in about a third of cases of carcinoma of the breast. Dietlen demonstrated pulmonary lesions secondary to sarcoma of the testis and bones, and secondary to hypernephroma, chorionepithelioma, malignant goiter, myosarcoma of the uterus, carcinoma of the breast and carcinoma elsewhere.