Although isolated lower motor neuron disease has been reported as a paraneoplastic complication, it has not been previously described, in association with anti-Hu antibody. We report a 51-year-old man in whom weakness heralded the presence of a small-cell cancer of the lung. His neurological disorder was characterized by an unremitting progression of limb, neck, and chest wall weakness and wasting that commenced and remained predominant in the upper limbs. Electrophysiological studies demonstrated widespread denervation and examination of a muscle biopsy specimen showed evidence of acute and chronic denervation. High titers of anti-Hu antibody were detected in the serum and cerebrospinal fluid. Neither objective measures of strength nor titers of anti-Hu antibody responded to corticosteroids, cyclophosphamide, intravenous immunoglobulins, or plasmapheresis. Death from the complications of motor neuron disease ensued 23 months after the onset of weakness. Autopsy revealed tumor in the lung and on pleural and peritoneal surfaces. There was a loss of anterior horn cells in the spinal cord. Despite the absence of symptomatic cerebellar disease, a decrease in the number of Purkinje cells was also detected.