Methods Consecutive patients presenting to ED for acute abdominal pain and with a diagnosis of pneumoperitoneum at CT or at surgery (study group) and a similar number of patients without a diagnosis of pneumoperitoneum (control group) undergo abdominal ultrasonography in a standardized protocol that include 10 scans for each patient registered on a video of 5 seconds. The videos are randomly reviewed by 4 sonographers and by 2 physicians with no experience in ultrasonography blind to final diagnosis and to all clinical data with the aim of detecting for each scan pneumoperitoneum. Accuracy of ultrasonography for the diagnosis of pneumoperitoneum will be calculated considering CT or surgery as gold standard. Furthermore intra and interobserver agreement and the accuracy of each ultrasonographic sign and scan will be calculated.