BACKGROUND Pneumoperitoneum usually is caused by hollow viscus perforation with associated peritonitis. Severe pneumoperitoneum secondary to infection of a hematoma with gas-forming organisms is exceedingly rare. METHODS Case report and literature review. RESULTS A 43-year-old man with a history of abdominoperineal resection for rectal cancer developed abdominal distention, fever, and elevated white blood cell count after lysis of adhesions with bowel resection for recurrent small bowel obstruction. Abdominal radiography and a computed tomography scan demonstrated a large amount of free air in the peritoneal cavity. Contrary to expectations, reexploration revealed no signs of viscus perforation or anastomotic leak, but instead a large pelvic hematoma with an odor was identified and evacuated. Cultures from the hematoma yielded anaerobic gram-negative bacilli (not Bacteroides fragilis). The patient recovered uneventfully. CONCLUSIONS Infected hematoma should be recognized as a cause of pneumoperitoneum after surgery. Awareness of this rare condition may prevent unnecessary surgical explorations in doubtful situations.