Spirometry and regional pulmonary function studies using xenon 133 gas were performed in 251 patients who had primary lung cancer. Surgical resection was undertaken in 150 while the remainder were treated with nonsurgical modalities. Pulmonary function studies were repeated postoperatively in 54 patients. Regional ventilation and perfusion of the tumor-bearing lung were decreased in patients with larger primary tumors and in those with involvement of ipsilateral hilar lymph nodes. Reduced regional function was also directly related to the proximity of the primary tumor to the hilum. Significant hypoperfusion did not contraindicate operation in 14 patients; however, 13 of them required pneumonectomy. Estimated postoperative forced expiratory volume in 1 second (FEV1.0), derived from preoperative spirometry and regional function of the tumor-bearing lung, correlated well with the measured postoperative values. These estimations were valuable in determining the extent of safe resection and correlated well with short-term survival. Long-term survival correlated better with the stage of disease.