Idiopathic brachial plexus neuropathy (BPN) is an immune-mediated disorder characterized by an acute onset of painful weakness in one or both upper extremities. The course is usually monophasic with gradual improvement over months; however, occasionally BPN can recur. Electrophysiologic studies suggest the pathogenesis is primarily axonal in the majority of cases. We describe an unusual case of BPN in which the patient had a chronic and relapsing course of painless weakness associated with conduction blocks and other electrophysiologic features of demyelination across the brachial plexus. The patient improved following treatment with intravenous immunoglobulin. The neuropathy falls within the spectrum of chronic inflammatory demyelinating polyneuropathy and multifocal motor neuropathy.