Perforation of the rectal malignancy has led to the extensive dissemination of gas throughout the adjacent soft tissues, tracking down the left thigh to the level of the knee. The finding of gas within soft tissues on a plain radiograph often raises the possibility of necrotizing fasciitis. In this case, the primary cause of the gas is perforation of a gas containing viscus, but secondary infection by gut microbes may lead to necrotizing fasciitis. Fascial thickening and enhancement are useful but inconsistent signs on CT in necrotizing fasciitis.