A 42 year old man with colonic Crohn’s disease presented to the orthopaedic department with septic arthritis of the knee with frank intra-articular pus, which persisted despite repeated joint washouts. This coincided with a symptomatic flare of his colonic Crohns. Imaging revealed a fistulous connection from the proximal sigmoid colon into the pelvis, a psoas abscess, myositis of the musculature around the left pelvic girdle and left thigh and two focal collections almost surrounding the distal femur. The knee symptoms only settled after laparotomy and resection of the diseased sigmoid colon and fistula. This case report demonstrates an unusual presentation of fistulating Crohns disease as septic arthritis.