Diagnosing hemothorax after blunt trauma may be aided by emergency department (ED) ultrasound (US). Various prior studies have evaluated ED US using different gold standards. A prospective study of blunt trauma patients who underwent computed tomography (CT) scan of the chest, abdomen, or both, was performed. Before CT scan, an US examination was performed specifically to identify free fluid in the thorax. The CT scan findings were used as the gold standard for validation of US results. From July 1998 to June 1999, 142 of 155 patients who underwent US and CT scan for evaluation of blunt trauma were included in this study. The CT scan identified 16 cases of hemothorax among these patients. ED US resulted in 2 true-positive, 2 false-positive, 14 false-negative, and 124 true-negative findings. ED US was 12.5% sensitive and 98.4% specific. ED US did not detect small-volume hemothorax identified by CT scan. Future research should focus on further defining the size of hemothorax appreciable with ED US, with increased attention paid to the type of gold standard implemented for its evaluation.