We report the case of a 48-year-old man who was admitted to the intensive care unit for acute respiratory distress. After discharge, the patient showed massive pneumoperitoneum. None of the complementary investigations provided information on the cause. The patient underwent surgery but no intraoperative diagnosis was reached. We believe that this case is of interest since pneumoperitoneum is a frequent entity that often leads to emergency laparotomy. The cause is usually perforation, although in a substantial proportion of cases the pneumoperitoneum is non-surgical, or idiopathic.