Pneumoperitoneum (PP) in artificially ventilated patients is usually caused by barotrauma to the lungs or by perforation of an intra-abdominal viscus. The differential diagnosis is sometimes difficult but important, as unnecessary laparotomy could result if the PP is due to pulmonary barotrauma. Three cases of PP are presented and the literature reviewed in order to demonstrate the source of the intra-abdominal gas, the factors predisposing to PP, the causes and mechanisms of PP, and its diagnosis and treatment. A classification based on the mechanism involved in the production of PP is presented.