We describe a patient with a chronic, asymmetric, limb weakness, fasciculations, relatively preserved reflexes, normal cranial nerves and no sensory abnormalities. The symptoms had been progressive over 2 years with a stepwise course, and then stabilized. Electrodiagnostic studies showed the presence of multifocal conduction blocks in motor nerves at sites not prone to compression; by contrast, the sensory conduction studies over the same nerve segments were normal, indicating selective involvement of motor fibers. Serum antibodies to the ganglioside GM1 were absent (ELISA). Treatment with prednisone failed to demonstrate any positive effect; a beneficial response was observed only with immunoglobulin therapy.