We report a 38-year-old man with a pure motor syndrome and IgM gammopathy leading to flaccid quadriplegia. Improvement followed treatment with dexamethasone, cyclophosphamide, and plasmapheresis, but he died of pulmonary embolism. At autopsy, he had a proximal motor axonopathy with lymphocytic infiltration of ventral roots. Proximal motor neuropathy may masquerade as motor neuron disease. The association with gammopathy and response to treatment suggest that patients with motor neuron disease should be routinely screened for serum protein abnormalities.