The appearance of a lung opacity on a chest film of a patient with known cancer may present a diagnostic dilemma. From 1940 through 1975, over 800 patients with this problem underwent thoracotomy for confirmation of diagnosis. In some 500 of these patients, the lesion proved to be primary cancer of the lung; in 196 they were solitary metastases and in 11 patients the lesions were benign. There were six additional patients in whom multiple opacities were found which proved to be benign conditions. An approach to the investigation, diagnosis, and surgical management of such solitary pulmonary lesions is presented. It is emphasized that the appearance of a solitary pulmonary shadow in a patient with a history of cancer should not be assumed to be a metastasis. Appropriate investigations should be performed without delay in an effort to define the nature of the lesion by microscopic analysis permitting definitive therapy to be administered and a more accurate prognosis provided.