An 82-year-old man presented with abdominal pain and a painful and swollen left thigh. On examination there was oedema, tenderness and crepitus with skin discoloration of the left thigh. Computed tomography showed retroperitoneal perforation of the caecum and necrotising fasciitis of the left thigh. A right hemicolectomy and repeated fasciotomies of the left thigh with debridement of necrotic tissue were performed but the patient died of multi-organ failure. Histology showed a pT4aN2c caecal adenocarcinoma. This is a unique presentation of a retroperitoneal caecal perforation and acts as a reminder that unexplained severe fasciitis of the thigh may be caused by an intra-abdominal pathology.