OBJECTIVES: The American College of Chest Physicians functional guidelines classify patients with predicted postoperative forced expiratory volume in 1 s or predicted postoperative carbon monoxide lung diffusion capacity <60% and with maximal oxygen consumption (VO2max) between 10 and 20 ml/kg/min in a heterogeneous category broadly defined as 'moderate risk' with variable morbidity and mortality. Data to support this statement are lacking. Using the European Society of Thoracic Surgeons database, our goal was to test this definition by evaluating the morbidity and mortality of those patients falling into this class. METHODS: All patients who had anatomical lung resection for lung cancer (2007-2019) and were deemed of moderate risk were identified in the European Society of Thoracic Surgeons database. Cardiopulmonary morbidity and 30-day mortality of these patients were assessed by the type of operation. RESULTS: A total 2016 patients were identified. The incidence of cardiopulmonary complications in this group was 21% after lobectomy (294/1435), 29% after bilobectomy (33/112), 22% after pneumonectomy (72/333) and 16% after segmentectomy (22/136) (analysis of variance P = 0.07). The 30-day mortality was 3.4% after lobectomy (49/1435), 8.9% after bilobectomy (10/112), 7.8% after pneumonectomy (26/333) and 3.7% after segmentectomy (5/136) (analysis of variance P = 0.0005). The 30-day mortality rate was 1.6-fold higher in patients with a VO2max between 10 and 15 ml/kg/min compared to those with a higher VO2max [49/861 (5.7%) vs 41/1155 (3.5%); P = 0.022]. For operations that were less extensive than a pneumonectomy and were performed by minimally invasive surgery, there was no difference in mortality between patients with a VO2max between 10 and 15 ml/kg/min and those with a higher VO2max [7/181 (3.8%) vs 11/272 (4.0%); P = 0.92]. On the other hand, after open surgery, the mortality of patients with a lower VO2max (10-15 ml/kg/min) was higher than that of those with a higher VO2max [26/501 (5.1%) vs 20/721 (2.8%); P = 0.034]. Linear regression adjusting for the extent and access of the operation confirmed that within the moderate-risk group a VO2max <15 ml/kg/min was associated with higher mortality (P = 0.028; odds ratio 1.61; 95% confidence interval 1.1-2.5). CONCLUSIONS: Morbidity and mortality rates found in this study are not negligible and reinforce the recommendation to ensure careful patient discussion and informed decision-making prior to lung cancer resection surgery.