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Rationale Retroperitoneal colonic perforation is a rare cause of retroperitoneal abscess. It presents, more frequently in frail elderly patients, with heterogeneous signs and symptoms which hamper the clinical diagnosis. Subcutaneous emphysema with pneumomediastinum and iliopsoas muscle abscess are unusual signs. Colonic retroperitoneal perforation may be consequent to diverticulitis or locally advanced colon cancer. Due to the anatomy of the retroperitoneal space and different physiopathology, diverticular perforation may present with air and pus collection; on the other hand perforated colon cancer may cause groin mass and psoas abscess. We reported 2 cases of colonic retroperitoneal perforation from diverticulitis and locally advanced colon cancer, respectively. Aim of this report is to improve differential diagnosis based on clinical signs. Patients' concerns A 71-year-old man presented with pain in his left side, fatigue, fever, nausea, massive subcutaneous emphysema of the neck, and Blumberg sign in the left iliac fossa. A 67-year-old man presented with abdominal pain, sub-occlusion, left groin mass, left groin, and lower limb pain during walking, negative Blumberg sign. Diagnosis In the first patient the computerized tomography revealed pneumoperitoneum, gas in the mesosigma, pneumomediastinum, wall thickening of the descending colon, and retroperitoneal collection from diverticular perforation. In the second patient abdominal CT scan found thickening of the sigmoid colon adherent to the iliopsoas and fluid collection. Interventions In the first patient, a left hemicolectomy extending to the transverse colon, followed by a toilette and debridement of the retroperitoneum were performed. In the second patient, tumor of descending colon perforated in the retroperitoneum with iliopsoas abscess was treated with left hemicolectomy and a drainage of the abscess. Outcomes The first patient underwent right colectomy with ileostomy in the 7 postoperative day for large bowel necrosis. He died of sepsis 2 days after. The second patient had regular postoperative and he is still alive. Lessons The spread of retroperitoneal abscess in complicated colonic diverticulitis is different from that in advanced colonic cancer. The former can present with a subcutaneous emphysema, the latter with a groin mass. Hence a thorough clinical examination and radiological studies are needed to diagnose these conditions.