Regional pulmonary function studies using 133xenon gas, spirometry, and arterial blood gas levels were performed before and 1 to 47 months after pneumonectomy for bronchogenic carcinoma in 27 patients. The mean loss in forced vital capacity was more after right pneumonectomy (44.9 percent of preoperative value) than after left lung resection (41.4 percent). There was no significant change in regional pulmonary function distribution within the remaining lung in 24 patients. Two patients developed significant changes in regional pulmonary blood flow; one had hepatic cirrhosis, and the other sustained a myocardial infarction after pneumonectomy. The third patient with significant apical hyperperfusion before pneumonectomy gradually developed abnormal distribution of ventilation concomitant with electrocardiographic evidence of cor pulmonale within two years after pneumonectomy. The mean ventilation of the apical zones was significantly lower than the mean of 14 healthy subjects. This finding and the higher incidence of ventilatory defects were related to old age and heavy smoking. Seven patients with marked reduction of pulmonary blood flow to the tumor-bearing lung (9 to 33 percent of cardiac output) had technically successful pneumonectomy. A formula and nomogram were developed to estimate the prognostically significant forced expiratory volume in one second after pneumonectomy from the preoperative studies.