From 1949 through 1972 at Memorial Sloan-Kettering Cancer Institute, 72 breast cancer patients developed a synchronous or metachronous solitary lung shadow. Forty-three had separate primary lung cancers; 23 had breast carcinoma metastases and six had benign lung lesions. Of these, 47% were asymptomatic and the lesions were discovered by routine chest roentgenograms. Among 49 patients who underwent pulmonary procedures, secondary involvement of regional lymphatics was noted in 22, nine of which were daughter metastases secondary to the breast deposit. In such a setting, a radical lobectomy (or pneumonectomy) would seem the operation of choice. Four patients who had primary lung cancer and five patients with pulmonary breast metastases survived five years after the thoracic procedure. Prolonged survival in these patients bore no relationship to age, sex, state of axillary lymph nodes or length of interlude between the breast and the lung cancer. Early diagnosis, the extent of the cancer and adequate excision seem to influence end results. When other primary cancers were present, in addition to those of breast and lung, these did not seem to diminish the chance for survival provided they were treated as if they existed alone. The need for strict periodic and continual followup examinations, not only in breast, but also for all cancer patients is essential for early detection of metastases or new primaries. Chest roentgenograms are an integral part of such a plan. The ambiguity of a solitary lung shadow (or at the most two or three opacities) with a known primary breast cancer requires clarification without delay as to its histologic nature so that proper treatment can be instituted and an accurate prognosis given.