Necrotizing infections are rapidly progressive potentially lethal bacterial diseases of the soft tissues. In based on the widely varying levels of soft tissues affected and the variety of the microflora, two types of necrotizing soft tissue infec tions need to be delineated: pure Chlostridial myonecrosis and other necrotizing soft tissue infections (NSTI). From an etiopathogenetic point of view NSTI can be secon dary to perianal or urogenital abscesses, traumatic lesions, wound infections, trophic or decubitus ulcers, oral cavity abscesses; only in a limited number of cases their origin can be idiopathic. Exceptionally it can happen that a NSTI could represent the only clinical manifestation of a retroperitoneal colic perforation. The Authors report their experience regarding two clinical cases recently observed. The first patient, who previously underwent colic resection for sigmoid carcinoma and adjuvant chemotherapy, had developed as only clinical manifestation of retroperitoneal anastomotic fistula a necrotizing infection at the root of the hip, extended along the whole leg. The second patient, with diverticular perforated disease, had developed rhe necroti zing infection in the lumbar region and in the perirenal tissues. Here will be discussed the sensitivity of the possible dia gnostic investigative techniques and the therapeutical stra tegies thet brought both the patients to a complete recovery.