Pneumomediastinum can be caused by gas dissecting along fascial tissue planes into the mediastinum from remote locations, including the retroperitoneum. One potential source of retroperitoneal gas is the colon. We present the third reported case of pneumomediastinum (plus pneumothorax and subcutaneous emphysema) without free intraperitoneal gas developing during an attack of ulcerative colitis. Because there was no colonic perforation noted at colectomy, the extracolonic gas was presumed to originate from a microscopic or partial thickness perforation of the colon. GI perforation must be considered not only in patients who have free intraperitoneal gas but also in those who present with symptoms, signs, or studies consistent with retroperitoneal gas, such as subcutaneous emphysema, pneumomediastinum, or pneumothorax.