Patients with acute brachial plexus neuritis are often misdiagnosed as having cervical radiculopathy. Acute brachial plexus neuritis is an uncommon disorder characterized by severe shoulder and upper arm pain followed by marked upper arm weakness. The temporal profile of pain preceding weakness is important in establishing a prompt diagnosis and differentiating acute brachial plexus neuritis from cervical radiculopathy. Magnetic resonance imaging of the shoulder and upper arm musculature may reveal denervation within days, allowing prompt diagnosis. Electromyography, conducted three to four weeks after the onset of symptoms, can localize the lesion and help confirm the diagnosis. Treatment includes analgesics and physical therapy, with resolution of symptoms usually occurring in three to four months. Patients with cervical radiculopathy present with simultaneous pain and neurologic deficits that fit a nerve root pattern. This differentiation is important to avoid unnecessary surgery for cervical spondylotic changes in a patient with a plexitis.