Lipoma arborescens is a benign tumor, but it may be a reactive process to other disorders, and its clinical, analytical, radiological and ultrasound presentation may be redundant to any synovial tumor. Despite the characteristic feature on magnetic resonance imaging (MRI), the correct differential diagnosis in atypical presentation, and the need for timely removal of the lesion to prevent joint damage, forces, ultimately, to invasive procedures. The clinical case reported here, fourth described in English language publications on the polyarticular form, also presented other specificities related to one of the swellings, in the knee. Because of its atypical location in the popliteal fossa, recurrent episodes of joint effusion, personal history of knee trauma, pulmonary tuberculosis, and family history of rheumatoid arthritis required particular attention. This process was hampered by the refusal of knee (and ankle) surgery by the patient. He accepted surgical removal of the swellings of the wrists, for aesthetical reasons, with pathologic confirmation of the diagnosis, and clinical success in that location. MRI of the knee showed the typical image of lipoma arborescens, but also other changes that compromise the prognosis.