Lipoma arborescens is a rare intraarticular lesion, mainly affecting the knee. A fourteen-year-old girl presented with a slow-growing painless mass in the right knee, of a six-year history. There were no limitations in the movements of the knee. Magnetic resonance imaging (MRI) revealed multiple lesions showing villous lipomatous proliferation of the synovium in the supra- and retropatellar regions and effusion in the knee joint. The mass was excised with arthrotomy and synovectomy and a histopathologic diagnosis of lipoma arborescens was made. Two years postoperatively, and when the right knee was completely asymptomatic, she developed a similar mass in the suprapatellar region of the left knee. An MRI scan showed a mass lesion in the supra- and retropatellar regions, effusion in the knee joint, and a synovial cyst in the popliteal fossa. Again, arthrotomy and synovectomy were performed and the histopathologic diagnosis was lipoma arborescens. This case differs from simultaneously involved knees in that lipoma arborescens in the contralateral knee developed two years after the initial operation.