Background Pneumothorax (PTX) is an emergency that requires urgent management to avoid catastrophic consequences. PTX is also an important cause of respiratory failure in the emergency department, and may occur frequently as a complication of central venous catheter insertion. Aim The aim of this study is to compare the diagnostic accuracy of bedside lung US with those for anteroposterior (AP) chest X ray (CXR) for the detection of PTX in critically ill patients. Methods This study was conducted on fifty adult patients from both sexes, mechanically ventilated at least 48 hours and planned for central line insertion. We excluded overt pneumothorax, patients requiring immediate invasive intervention, pregnancy and lactation. Lung ultrasound was done to all patients after 30 minutes from central line insertion followed by CXR to confirm the diagnosis of pneumothorax. Pneumothorax was confirmed using CT chest. Results Results showed that ultrasound is superior to chest X Ray in detection of PTX.Ultrasound showed sensitivity of 94.87%, specificity of 81.82%, positive predictive value of 94.87%, negative predictive value of 21.82% and accuracy of 92.0% in detection of PTX, while Chest X Ray showed sensitivity of 76.92%, specificity of 63.64%, positive predictive value of 88.24%, negative predictive value of 43.75% and accuracy of 74.0% in detection of PTX. Conclusions In conclusion, US represent a good approach for the evaluation of PTX, with advantages of timeliness, high accuracy and high reliability.