A series of 186 patients with blunt chest trauma was studied with transthoracic ultrasonography to diagnose pneumothorax and to evaluate its size and location. The results were compared with bedside chest radiography and spiral CT scan. The prevalence of pneumothorax on CT scan was 56/186 (30.1%). Pneumothorax was proven on radiography in 30/56 cases without false positive results: "radiographic deep sulcus sign" was evident in 3/29 cases, 26/29 cases being occult. The ultrasound study demonstrated the presence of pneumothorax in 55/56 patients: one occult pneumothorax was missed and no false positive results were observed. The CT scan differed of +/-2.3 cm (range 1-5 cm) from the US study in evaluating size and location of pneumothorax. In conclusion, ultrasound study may detect occult pneumothorax undiagnosed by standard plain radiography. It reflects accurately the extent of pneumothorax if compared with CT scan, outlining the "ultrasonographic deep sulcus sign" on anterior chest wall.