We evaluated 33 high-risk patients before pneumonectomy, all of whom had a forced expiratory volume in one second (FEV1) of less than 2.0 L before surgery. A quantitative perfusion lung scan was used to assess the right-left distribution of blood flow. A predicted postoperative FEV1 was calculated from the information on the lung scan and the preoperative FEV1. If this calculated value exceeded 800 ml, the patient was physiologically cleared for surgery up to and including a pneumonectomy. Surgery was otherwise believed to be contraindicated in the absence of studies using balloon occlusion. Perioperative mortality (less than or equal to 30 days after surgery) was found to be 15 percent (5/33). In surgery of this magnitude, we find this to be an acceptable percentage of mortality and have continued to use these simple physiologic criteria to determine whether a patient can tolerate pneumonectomy.