This is the first report of brachial plexus inflammation associated with clinical herpes zoster paresis. A 78-year-old female with a 3-week history of herpes zoster of the C4, C5, and C6 dermatomes developed left upper arm monoplegia. She died from an acute myocardial infarction. Post-mortem provided a rare opportunity to study the neuropathology of herpes zoster motor involvement. Histology of the brachial plexus showed extensive lymphocytic infiltration, myelin breakdown, and preservation of axons without vasculitis. The cervical spinal cord showed perivascular lymphocytic cuffing and no anterior horn necrosis. We suggest, the brachial plexus inflammation was a distal extension of a dorsal ganglionitis. Brachial plexus neuritis may be a direct cause of reversible upper limb paresis in herpes zoster. We demonstrate the motor neuropathy is an inflammatory demyelinative process consistent with the recovery observed in a number of patients. We postulate post-herpetic neuralgia may be related to an ongoing inflammatory process.