Forty patients were subjected to multi-stage exercise tolerance tests using a bicycle ergometer prior to thoracotomy. During the test, expiratory gases were analyzed, and the arterial blood gas level and lactic acid level were studied. Postoperative complications were found in 10 of 40 cases (25%). Significant differences between patients with and patients without postoperative complications were recognized in VO(2)max, VCO(2)max and PaO(2)max. Patients with a VO(2)max value less than 1100 ml/min after the exercise test with a bicycle ergometer had a high risk of developing postoperative complications. Twelve postthoracotomy patients underwent the same test. All of their VO(2)max values were significantly lower than the corresponding preoperative values.