Lipoma arborescens is a rare intraarticular lesion of unknown etiology. We describe the radiographic, computerized tomographic, and magnetic resonance (MR) imaging characteristics of lipoma arborescens in a 17-year-old boy presenting with chronic painful swollen knee. Histologic examination after surgical synovectomy confirmed the diagnosis of lipoma arborescens. The fatty nature and frond-like appearance of this lesion can be demonstrated on imaging studies, especially MR imaging. Open synovectomy is curative in most cases. Although lipoma arborescens is rare, it is important to recognize and differentiate it from other synovial lesions by MR imaging.