Sarcoma arising in the brachial plexus is a rare complication following radiation therapy, with only 7 patients reported in the literature. The absolute number of patients is likely to increase with the growth of radiation therapy to treat breast cancer. Because of its rarity, early diagnosis requires a high degree of suspicion. We present a patient with neural sheath sarcoma of the brachial plexus arising 18 years after external beam radiation for breast cancer. The interval between presentation and definitive therapy was prolonged by workup and treatment of carpal tunnel syndrome and cervical nerve root compression. The English language literature regarding postradiation sarcoma is reviewed and the clinical, radiological, and electrophysiological workup of postradiation brachial plexopathy is discussed.