Intra-abdominal free air represents the collection of air in the peritoneal cavity (pneumoperitoneum) and/or in the retroperitoneum (pneumoretroperitoneum). Pneumoperitoneum mainly indicates the perforation of an intra-abdominal viscus, but in about 10% of cases, a nonsurgical source is responsible for the presence of free air in the peritoneal cavity. Pneumoperitoneum is produced by the rupture of a hollow viscus including the stomach, the small bowel, and the large bowel, with the exception of duodenal and colonic segments that are retroperitoneal. The most frequent source of spontaneous pneumoperitoneum is the perforation of a gastric or duodenal ulcer. Prompt diagnosis of an alimentary tract perforation, together with detection of the site and cause, can speed up treatment and improve prognosis, having a large impact on therapeutic management, including a focused conservative treatment or the type of surgery. Multidetector row computed tomography is highly sensitive for detecting the presence of free extraluminal air. It can demonstrate extraluminal, intraperitoneal, or retroperitoneal air more sensitively in comparison with plain radiographs. In addition to discovering the presence of intra-abdominal free air, multidetector row computed tomography can also determine the correct cause of perforation and localize the perforation site.