Lipoma arborescens is a rare, benign intra-articular hyperplastic tumor characterized by villous, polypoidal, and lipomatous proliferation of the synovium. The knee is the most commonly affected joint1,2. Intra-articular lipoma arborescens is rare in adults3-7 and extremely rare in children5,8-11. The disease is typically considered to be a unilateral condition, and bilateral disease has been reported to be extremely rare, especially in children8-13. Patients usually present with chronic or recurrent painless swelling of the knee joint. Lipoma arborescens must be considered in the differential diagnosis of chronic joint effusions in childhood10. Our patient and his family were informed that data concerning the case would be submitted for publication, and they consented. A thirteen-year-old boy was admitted to our hospital because of chronic pain in the left knee. He had no history of trauma or infection. The left knee had been painful for four weeks during walking and running, thus limiting his daily activities. There were no locking symptoms. The patient’s medical history and the family history were unremarkable. The boy was healthy and had a body mass index of 21 kg/m2. Physical examination revealed bilateral swelling of the knee joints. Both knees had a normal local temperature and normal skin coloration and were not tender to palpation. Both joints were more voluminous than expected, and a mild joint effusion was present bilaterally. Full extension and 140° of flexion were possible in both knees, and knee motion did not elicit pain. There was no tissue mass around the knees, and no laxity could be detected. The results of blood tests, including measurements of the erythrocyte sedimentation rate, rheumatoid factor, C-reactive protein level, histocompatibility antigen B27 (HLA-B27), and antinuclear immunoglobulin, were normal. …