Brachial plexus neuritis is a rare neurologic disease that may be overlooked in emergency medicine because other conditions are much more common. We report a case of brachial plexus neuropathy due to cytomegalovirus infection. The diagnosis was based on history, clinical findings, laboratory tests, and electromyography. Early diagnosis and adequate treatment is important to avoid unnecessary investigation, prevent complications (especially adhesive capsulitis of the shoulder), and reassure the patient.