A solitary pulmonary nodule appearing in a patient with breast cancer, either past or present, is most likely to be a second primary cancer originating in the lung rather than a metastasis from the breast cancer. Between 1970 and 1983 there were at this institution 1416 patients with breast cancer and 579 patients with bronchogenic cancer, 198 of whom were women. Among the patients with breast cancer, 42 (or 3% of all of the patients with breast cancer) had a solitary pulmonary nodule either at the time of presentation of their breast cancer or during the follow-up period. Fifty-two percent of the solitary pulmonary nodules proved to be a primary lung tumor, 5% proved to be benign lesions, and only 43% proved to be metastatic breast cancer. Patients with breast cancer with solitary pulmonary nodules should have a diagnostic workup appropriate for lung cancer. Since adenocarcinoma has become the most common lung cancer cell type, the usual diagnostic tests may not allow a firm differentiation between primary lung and secondary breast cancer. Therefore if malignancy is proved or suspected, thoracotomy with appropriate resection is the treatment of choice in most patients with breast cancer, even at the initial appearance of the breast cancer.