Transthoracic ultrasound (US) is useful in the evaluation of a wide range of peripheral parenchymal, pleural, and chest wall diseases. Furthermore, it is increasingly used to guide interventional procedures of the chest and pleural space. The role of chest US in the diagnosis of pneumothorax has been established, but comparison with lung computed tomography (CT) scanning has not yet been completely performed. The purpose of this study is to prospectively compare the accuracy of US with that of chest radiography in the detection of pneumothorax, with CT as the reference standard. One hundred ninety-seven patients who were evaluated by spiral chest CT scan for various clinical indications were prospectively evaluated. Ultrasonography was performed by a radiologist, blinded to the chest CT findings. Sensitivity, specificity, and accuracy of ultrasound in the detection of pneumothorax were then compared with chest CT scan. CT scan showed pneumothorax in 92 patients. Sonography and plain X-ray of the chest revealed 74 and 56 cases of pneumothorax, respectively. Statistical analysis disclosed the US to be 80.4 % sensitive and 89 % specific in the detection of pneumothorax with an overall accuracy of 85 %. In this study, US was more sensitive than chest radiography in the detection of pneumothorax. The results of this study suggest that thoracic US, when performed by trained individuals, can be helpful for the detection of pneumothorax.