Although severe impairment on routine pulmonary function tests will identify patients with a high post-thoracotomy morbidity, cardiopulmonary complications often develop in patients with only a mild-to-moderate impairment in pulmonary function. To determine whether the preoperative exercise capacity can prospectively identify those at risk of developing complications, 22 patients scheduled for thoracotomy (mean age, 55.7 +/- 2 yr) underwent an incremental exercise test on a cycle ergometer to determine their maximal O2 uptake (VO2max) prior to thoracotomy. Routine pulmonary function tests were performed and postoperative forced expiratory volume in one second (FEV1) was predicted from split function perfusion lung scan in all subjects. Eleven of the 22 patients had no cardiopulmonary complications postoperatively. The age, history of prior cardiovascular disease, degree of impairment on routine pulmonary function tests, and predicted postoperative FEV1 were similar in those who did and those who did not experience complications. However, those without complications had a significantly higher VO2max than did those who experienced complications (22.4 +/- 1.4 versus 14.9 +/- 0.9 ml/kg/min, p less than 0.001). Only 1 of 10 patients with a VO2max greater than 20 ml/kg/min had a complication, whereas all 6 patients with a VO2max less than 15 ml/kg/min had a complication. We conclude that exercise testing is a useful adjunct in the evaluation of operative risk for thoracotomy.