Brachial plexopathy is a common and disabling complication in cancer patients most often attributed to metastasis or radiation-induced fibrosis. Occasionally, other rare but potentially treatable causes are found. A 73 year old woman had a left radical mastectomy followed by radiation to the chest wall and axilla 24 years ago. She recently presented with left arm pain, chronic, nonprogressive lymphedema, profound distal arm sensory loss and progressive severe hand weakness. There was moderate atrophy of all intrinsic hand muscles, anesthesia of the hypothenar eminence and 4th and 5th digits, and no adenopathy or palpable mass in the axilla. EMG confirmed a brachial plexopathy. MRI showed loss of tissue planes consistent with radiation fibrosis, but CT showed a discrete mass in the brachial plexus. Open biopsy showed pleomorphic spindle shaped cells with immunoperoxidase stains consistent with malignant fibrous histiocytoma. Radiation-induced malignant fibrous histiocytoma may present with a brachial plexopathy in the absence of a palpable mass and should be considered in the differential diagnosis of brachial plexus lesions in cancer patients. CT scanning through the plexus may be useful when MRI is normal or equivocal.