The case study in this article describes the rapid and accurate diagnosis of a critically ill patient with necrotising fasciitis (NF). Full-thickness patchy skin necrosis of the right thigh, buttock and flank was detected on admission. Prompt radical debridement together with aggressive fluid resuscitation and broad-spectrum antibiotic administration was initiated. Case ascertainment was used to evaluate the effectiveness of a debridement and wound treatment regime, using a monofilament debridement product, negative wound pressure treatment and, after the critical period had ended, a bio-cellulose + polyhexamethylene biguanide (PHMB) dressing, followed by a collagen dressing. NF after open haemorrhoidectomy represents a life-threatening complication to otherwise healthy patients. Accurate diagnosis, prompt critical care and surgical treatment, together with debridement using the monofilament product and effective wound bed preparation, lead to a successful outcome.