The findings in 28 patients with synchronous lung cancers are reviewed. Mediastinoscopy and systemic staging were performed to exclude the possibilities that one pulmonary lesion was metastatic from the other or that both represented systemic metastases from another tumor. Nineteen patients underwent resection of both tumors. Median survival was 25 months for four patients with definite Stage I synchronous cancers (no nodal involvement; different cell types, bronchoscopically separate endobronchial lesions or arising from separate foci of carcinoma in situ) and was 27 months for seven patients with possible synchronous Stage I cancers (no nodal involvement; similar cell types; located in separate lobes). Median survival was 11 months for 16 patients having Stage II or III lung cancer accompanied by a second synchronous lung cancer. In the absence of hilar or mediastinal nodal involvement and systemic metastases, synchronous tumors should be considered separate primaries when located in different lobes, even if they have similar histologic features. Prognosis of synchronous cancers is related to the presence or absence of nodal metastases. Pneumonectomy is the operation of choice for synchronous unilateral tumors. With bilateral tumors, sequential resection starting with the most advanced lesion is appropriate. Preservation of lung tissue without compromising the cancer operation is critical.