Necrotising fasciitis (‘synergic gangrene’) is a serious bacterial soft tissue infection with a rapidly progressive course and high mortality. It is caused by a variable combination of obligate anaerobes, streptococci, staphylococci, and Gram-negative bacilli. Histologically, it is characterised by obliterative endarteritis and thrombosis of the subcutaneous vessels, leukocyte infiltration, and fascial necrosis.1It predominantly occurs in patients who are debilitated by a co-morbid condition such as diabetes, alcoholic liver disease, or intravenous drug use. We have described a case of necrotising fasciitis secondary to a locally advanced carcinoma of the lower rectum, as a likely consequence of perforation into the surrounding extraperitoneal tissues. The patient had no other identifiable predisposing factors.