The objective of this study was to investigate the feasibility of using ultrasound (US) in place of portable chest x-ray (CXR) for the rapid detection of a traumatic pneumothorax (PTX) requiring urgent decompression in the trauma bay. All patients who presented as a trauma alert to a single institution from August 2011 to May 2012 underwent an extended focused assessment with sonography for trauma (FAST). The thoracic cavity was examined using four-view US imaging and were interpreted by a chief resident (Postgraduate Year 4) or attending staff. US results were compared with CXR and chest computed tomography (CT) scans, when obtained. The average age was 37.8 years and 68 per cent of the patients were male. Blunt injury occurred in 87 per cent and penetrating injury in 12 per cent of activations. US was able to predict the absence of PTX on CXR with a sensitivity of 93.8 per cent, specificity of 98 per cent, and a negative predictive value of 99.9 per cent compared with CXR. The only missed PTX seen on CXR was a small, low anterior, loculated PTX that was stable for transport to CT. The use of thoracic US during the FAST can rapidly and safely detect the absence of a clinically significant PTX. US can replace routine CXR obtained in the trauma bay and allow more rapid initiation of definitive imaging studies.