Necrotizing fasciitis is a Potentially life-threatening infection that may resemble extensive pyoderma gangrenosum. The treatment of the two diseases is, however, different, and differentiating them is therefore essential. A case is presented in which necrotizing fasciitis appeared as a complication of pyoderma gangrenosum. The successful treatment is described and the differential diagnosis of the two entities is presented. The need for early and aggressive treatment of necrotizing fasciitis is stressed.