Lipomatosis of nerve is a rare, ill-defined, fibrofatty tumor intermingled among nerve fascicles. The classic presentation of lipomatosis of nerve describes focal distal involvement of the median nerve at the wrist. The purpose of this study was to determine the level of proximal lesion extension in cases of upper extremity lipomatosis of nerve using magnetic resonance imaging. The authors' study confirms that lipomatosis of nerve is more than a focal distal disease. Lipomatosis of nerve has the potential for multiple nerve involvement and for proximal nerve involvement extending to the nerve roots. Magnetic resonance imaging of the entire upper extremity may be indicated in patients with lipomatosis of nerve, especially when the ulnar nerve is affected.