Retroperitoneal colonic perforation in patients with ulcerative colitis is rare. We report such a case in a patient with severe ulcerative colitis without toxic dilatation in whom mediastinal and subcutaneous emphysema also developed. Unlike previously reported cases, our patient was treated conservatively with intravenous fluids, parenteral nutrition, intravenous hydrocortisone, and antibiotics. After 2 weeks, the mediastinal and subcutaneous emphysema and the retroperitoneal air completely disappeared.