Two cases of primary osteosarcoma of the lung are presented. In one case, the radiologic, clinical, and cytologic findings led to a preoperative diagnosis of undifferentiated carcinoma of the lung. In the second case, a lung nodule was discovered during postchemotherapy follow-up in a patient with lymphoma. Fine needle aspiration in the second case showed lymphoma, and further chemotherapy was instituted; however, persistent growth of the nodule prompted a resection. Microscopic examination of the resected tumors in both cases revealed histologic features of high-grade osteosarcoma. Flow cytometric analyses of the primary tumors showed abnormal hyperdiploid deoxyribonucleic acid populations in accordance with those seen in high-grade malignant neoplasms. Immunohistochemical studies supported a mesenchymal origin for these tumors. These tumors shared clinical features with other reported cases of primary osteosarcoma of the lung such as large size at diagnosis, occurrence in older individuals, and aggressive behavior.