Spontaneous pneumoperitoneum may pose a diagnostic dilemma, especially when evaluating a patient who has no or minimal abdominal or constitutional findings accompanying its presence. The free intraperitoneal air may be the result of a perforation of a hollow viscus in such instances. Under these circumstances, the air is most often from another source. The more common sites of origin are intrathoracic sites, air in the wall of the intestinal tract and the genital organs in women. Diagnostic and therapeutic procedures in or adjacent to the abdomen may result in an iatrogenic pneumoperitoneum. In each instance when the findings of an acute abdominal catastrophe are lacking, the circumstances present before the discovery of the pneumoperitoneum should give a clue to the appropriate diagnosis. When a reasonable nonsurgical cause can be discerned, continued observation may be sufficient, thus avoiding an unnecessary laparotomy.