Neoplastic plexopathy is an unusual, but known cause of neuropathy. This is a case report of a patient, who developed brachial plexopathy years after completing treatment for breast cancer. The patient presented with pain in one arm and, subsequently, developed sensory deficits and muscle atrophy, resulting in severely impaired hand function. Repeated magnetic resonance imaging and positron emission tomography scans were necessary before the final diagnosis could be made. The case report illustrates the importance of a thorough and repeated diagnostic work-up, when metastatic cancer is a perhaps unlikely but possible cause of plexopathy.