We report the results of immunosuppressive treatment with intravenous cyclophosphamide in 12 patients with lower motor neuron syndrome and elevated titers of serum autoantibodies to GM1 ganglioside. All patients had lower motor neuron dysfunction including proximal or distal weakness, fasciculation and muscle atrophy, but no upper motor neuron dysfunction such as hyperreflexia, spasticity or Babinski's sign. Electrophysiological studies revealed no evidence of conduction block, but EMG findings of acute or chronic denervation in the limbs were present. Serum biochemistry and immunological studies were negative for M protein. After a 6-month follow-up, despite a fall in antibody titer, there was no significant clinical improvement in any of the patients.