Eosinophilia of synovial fluid is an uncommon condition. The majority of the reported cases are associated to diseases such as rheumatoid arthritis, parasitic disease, hypereosinophilic syndrome, Lyme disease, and allergic processes as well as hemarthrosis and arthrography. Presently there are only four cases of eosinophilic synovitis with unknown cause. We are reporting a patient with oligoarthritis of the knees, massive eosinophilia, and Charcot-Leyden crystals in synovial fluid without associated cause. We review the clinical and biological features of eosinophilic synovitis and discuss its pathogenesis.