Idiopathic brachial plexitis is a rare disorder presenting with pain and weakness in the shoulder girdle and upper extremity. Idiopathic brachial plexitis can mimic other conditions that cause acute pain and weakness around the shoulder, and its diagnosis can be challenging. There is no special test for the diagnosis of idiopathic brachial plexitis, although electromyography may be useful. In this case of idiopathic brachial plexitis, we present magnetic resonance neurography findings for the first time. (c) 2005 by American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation