BACKGROUND Peripheral or cranial nerve root dysfunction secondary to invasion of the CNS in multiple myeloma is a rare clinical event that is frequently mistaken for other diagnoses. We describe the clinical utility of 18F-fluorodeoxyglucose positron emission tomography (FDG-PET)/CT scanning for diagnosing neuro-myelomatosis. CASE REPORT A 63-year-old woman whose chief complaints were right shoulder and upper extremity pain underwent MRI and 18F-FDG PET/CT scan. MRI revealed a non-specific brachial plexus tumor. 18F-FDG PET/CT demonstrated intense FDG uptake in multiple intramedullary lesions and in the adjacent right brachial plexus, indicating extramedullary neural involvement associated with multiple myeloma, which was confirmed later by a bone marrow biopsy. CONCLUSIONS This is the first reported case of neuro-myelomatosis of the brachial plexus. It highlights the utility of the 18F-FDG PET/CT scan as a valuable diagnostic modality.