Neurolymphomatosis typically appears as a diffuse lesion with thickening of the affected nerves on magnetic resonance imaging (MRI). MRI in the present case revealed a well-defined, solitary lesion showing continuity with brachial plexus nerves. Findings including clinical symptoms resembled benign nerve sheath tumour rather than neurolymphomatosis. Intra-operatively, the C8 root was focally swollen, corresponding to a well-circumscribed lesion on MRI. The diagnosis of neurolymphomatosis was obtained only after resection biopsy. Post-operatively, (18)F-fluorodeoxyglucose positron emission tomography proved useful for follow-up evaluation. We offer the first description of the MRI characteristics of brachial plexopathy in neurolymphomatosis, along with the clinical course.