SUMMARYA 75-year-old man presented in shock secondary to septic arthritis of the knee. The patient, with a known history of knee osteoarthritis, was treated elsewhere for mechanical locking symptoms and effusion with arthroscopic debridement, and developed septic arthritis, which disseminated to the leg and foot after a tear in the capsule, and a ruptured pyogenic popliteal cyst. Open debridement of the knee joint, and drainage of the abscesses of the leg and foot, were performed. Antibiotic-loaded cement beads were left in the residual space. Debridement was repeated and cement beads removed after 4 days, and finally the infection was eradicated without any serious consequences for the patient. There is debate over arthroscopic intervention for osteoarthritic knees. The presence of a popliteal cyst, which is a rather common finding in the latter, could be related to a significant number of complications, such as septic arthritis. BACKGROUND