Pneumoperitoneum could be due to life threatening conditions and its quickly diagnosis is important in the emergency department (ED). Signs and symptoms are non-specific and radiography has low accuracy in the acute setting. Computed tomography (CT) is considered the gold standard, however it is not a cost-effective option in the vast population of patients with abdominal pain. Ultrasonography is able to detect as little as 2 ml of free air, however diagnostic accuracy of ultrasonography remains unclear. This study evaluates the accuracy of ultrasonography for the diagnosis of pneumoperitoneum, the most accurate abdominal scan and the most accurate echographic sign for the detection of free air.