Background: A pneumothorax (PTx) which is a common problem in the ICU is defined as a collection of air outside the lung, within the pleural cavity, between the parietal and visceral pleurae, applying pressure on the lung and make it collapse. There are two types of pneumothoraxes, traumatic and atraumatic. The diagnosis of pneumothorax in ICU patients is important by using the least invasive techniques without patient transfer by using the classic sonographic findings in the form of: the lung sliding, the lung pulse, the B lines and the lung point. The first three signs are strongly predictive of absent pneumothorax; while the lung point is the sign that confirms a pneumothorax. The aim of our study is to assess role of chest ultrasound in diagnosis pneumothorax in ICU patients. Methods: The present study is a prospective study that was conducted on 50 patients with clinical data or history that suspect pneumothorax disease, who were admitted to critical care unit at Banha University Hospitals. All cases in our study were subjected to full history taking, full clinical examination and full investigations. Results: Forty-two patients (84%) had confirmed diagnosis of pneumothorax with CT. Ultrasound detected pneumothorax in 70% of the patients, while Chest X-ray detected pneumothorax in more than one-third of the patients (40%). Chest x-ray showed a sensitivity of 45.2%, a specificity of 87.5%, PPV of 95%, NPV of 23.3%, and an overall accuracy of 52%. Chest ultrasound showed a sensitivity of 81%, a specificity of 87.5%, PPV of 97.1%, NPV of 46.7%, and an overall accuracy of 82%. Regarding the degree of agreement with CT, X-ray showed poor agreement (Kappa = 0.155), while ultrasound showed moderate agreement (kappa = 0.505). Conclusion: lung ultrasound is an adequate diagnostic modality in ICU population to detect pneumothorax with more sensitivity than CXR, it is easy cheap and best bed side test with low exposure to ionized radiation compared to CT chest and CXR