This is a report of a case of nontraumatic posterior knee dislocation following septic arthritis in a 59-year-old male with leucopenia secondary to sarcoidosis. After confirmation of the diagnosis with synovial fluid analysis, the patient was taken emergently to the operating room for arthroscopic-assisted irrigation and debridement. Arthroscopy demonstrated intact cruciate ligaments but confirmed the diagnosis of septic arthritis demonstrating purulent synovial fluid. He was discharged after multiple repeat irrigation and debridement procedures and intravenous antibiotics improved his symptoms. The infection appeared to resolve as his laboratory values normalized following treatment. Three months later, however, x-rays revealed a nontraumatic posterior knee dislocation. We hypothesize that the soft tissues including the cruciate and collateral ligaments were made incompetent by enzymatic digestion and stretching with mobilization which allowed the knee to dislocate without a traumatic event.