Synovitis is the inflammation of the synovial membrane with unknown etiology which occurs in association with auto-immune inflammatory arthritis, mainly in rheumatoid arthritis. Synovitis manifesting as rapidly progressing monoarticular or pauciarticualr symptoms could make early diagnosis difficult, thus it could be misdiagnosed as other forms of arthritic diseases. We experienced a rare case of knee joint synovitis which initially manifested as mimicking a septic arthritis. A 58-year-old-male patient underwent renovascular embolization due to retroperitoneal hemorrhage which was developed after renal biopsy. Suddenly, the patient’s left knee joint became swollen rapidly with redness and tenderness. Moreover, his right knee also became inflamed. Surgical irrigation and intravenous antibiotics had never worked on his knee joint inflammation, however administration of intermediate dose of steroid could decrease inflammatory signs dramatically. Synovitis in a large joint could be mistaken as a septic arthritis, delaying the right diagnosis. Thus, we report this case with literature review. (J Rheum Dis 2015;22:39-44)

Keywords: Synovitis, Rheumatoid arthritis, Septic arthritis