Introduction: Brachial diplegia is a clinical term used to describe weakness restricted to the upper extremities. We report a case of brachial diplegia associated with West Nile virus infection. Methods: A 48-year-old man developed severe painless bilateral upper extremity weakness within a few weeks of a flu-like illness. Results: Clinical examination revealed marked periscapular, shoulder girdle, and humeral muscle atrophy and bilateral scapular winging, with near symmetrical bilateral hypotonic upper extremity weakness. This was associated with clinical signs of an encephalomyelopathy without cognitive or sensory deficits. Electrophysiological studies demonstrated a subacute disorder of motor neurons, their axons or both, involving the cervical and thoracic myotomes, with ongoing denervation. Serological studies confirmed recent West Nile virus (WNV) infection. Gradual improvement occurred following conservative supportive therapies. Conclusions: Progressive brachial diplegia is a rare neuromuscular presentation of WNV neuroinvasive disease. This case report adds to the clinical spectrum of WNV-induced neurologic sequelae. © 2012 Wiley Periodicals, Inc.