We present the case a 53-year-old patient followed-up since 1999, for erosive AR treated with methotrexate and glucocorticoids. In April 2000, he had an arthritis of the right knee. The identification of an enterobacter in blood culture, and synovial biopsy results permitted the diagnosis of septic arthritis. After 23 days of antibioterapy treatment, the patient had an arthritis of the left knee. The infectious origin was confirmed by synovial biopsy. The course was better after adaptation of the antibiotics. Septic arthritis is then a serious complication of AR. It requires a fast and multidisciplinary management. It can be threatenig in fragile and immunocompromised patients. The functional prognosis is especially compromised in polyarticular septic arthritis.