Background: Lung separation is the basis of thoracic anesthesia, which is performed by different instruments. Checking probable malpositioning of tracheal tube needs fiberoptic bronchoscopy. The aim of this study was to compare respirator suggested compliance with fiberoptic findings in detecting major tracheal tube malpositioning.Methods: A total of 256 patients undergoing thoracic surgery with double-lumen tracheal tube insertion in Imam Khomeini Hospital, Tehran, Iran, during 2010-11 were divided into three groups (n=86). We used left-sided double-lumen tube (DLT) for left or right-sided surgeries (groups 1 and 2), and right-sided DLT for left-sided surgeries (group 3). The position of the tubes was evaluated and compared using bag compliance versus fiberoptic bronchoscopy.Results: The mean age of the study population was 44.7±13.4 (16-73) years, while 155 (59.9%) were male. The sensitivity, specificity, positive and negative predictive values, and the accuracy of bag compliance test for left-sided DLT in supine position were 40% (95% CI: 20-60%), 99% (95% CI: 96-99%), 84% (95% CI: 54-94%) 92% (95% CI: 88-95%) and 92% (95% CI: 87-95%), respectively. The above-mentioned variables for lateral decubitus position respectively were 27%, 98%, 76%, 89%, and 88%. Malpositioning was more prevalent in right-sided DLTs (P=0.02).Conclusion: Based on the results of this study, and the high specificity, positive predictive value, and accuracy of bag compliance test, its use is encouraged as an alternative to fiberoptic bronchoscopy for checking DLT position, specially, in emergent surgeries or when fiberoptic bronchoscopy is unreachable due to lack of expertise or personnel.