Synovial lipoma arborescens is a rare and benign intra-articular pathology, of unknown etiology, characterized by a villous and lipomatous proliferation of synovial tissue. It presents with atypical clinical manifestations, usually located in the knee, represented as recurrent joint effusions and painless swelling joint. The magnetic resonance is the most specific test and can often even avoid the synovial biopsy. We related the case of a female patient with mechanical pain in the knee with indolent evolution for 18 years, clinical and radiological compatible with osteoarthritis. With the finding of a localized unilateral increase of the suprapatellar bursa without perceptible joint effusion and ultrasonographic aspect of an exuberant nodular synovitis, the possibility of villonodular pigmented synovitis had to be discarded by synovial biopsy. Even after this procedure, her diagnosis was not clear, being reported to rheumatology evaluation due to histopathology findings confused with rheumatoid arthritis. The set of clinical, laboratory, magnetic resonance and histological review of synovial tissue confirmed the diagnosis of synovial lipoma arborescens, excluding the possibility of rheumatoid arthritis.