This case report describes an acute colonic diverticular perforation occurred to a multiple myeloma patient, taking corticosteroid and morphine therapy, revealed by a subcutaneous emphysema of upper chest and right abdomen as initial presentation. Sigmoid diverticulitis with perforation and generalized peritonitis is a severe complication of the diverticular disease and it is due to diverticular microperforation. This condition occurs more frequently in patients with widespread diverticolosis and usually after 50 years of age, and the frequency of related complications increases with age (and with the use of corticosteroids). Extraperitoneal air from the sigmoid-rectum perforation can escape diffusing superiorly though paravertebral retroperitoneal tissues and via the diaphragmatic iatus into the mediastinum, producing pneumomediastinum and it diffuses to yield superior thoracic emphysema. This report suggests that the diagnosis of retroperitoneal perforation is usually difficult because of the lack of signs of peritoneal irritation and the paucity of symptoms, particularly in patients treated with corticosteroids.