A 61 year old man presents with slowly progressive balance difficulties and numbness in the lower limbs. Neurological examination is consistent with a sensory ataxia. Investigations reveal positive anti-GD1b antibodies with modestly elevated CSF protein. EMG and nerve conduction studies demonstrate a mild large-fibre axonal motor-sensory neuropathy with no demyelinating features. MRI reveals bilateral thickening of the lower brachial plexus with possible enhancement of affected nerves. Lower limb SSEPs are undetectable and prove to be the key investigation in localising the region of pathology to make a diagnosis.