We report a patient with asymmetrical patchy weakness of the limbs, and with autoantibodies against gangliosides GM1, GD1b, asialo GM1. Although electrophysiological studies did not reveal conduction block, treatment with prednisolone resulted in clinical improvement. A 52-year-old man was admitted to Kyoto University Hospital, because of gait disturbance. Neurological examination revealed a patchy distribution of weakness in the limbs. Deep tendon reflex was normal at the right knee, and was depressed at the right biceps. Other deep tendon reflexes were absent. There was a slight decrease in vibratory sensation in the distal portions of the lower extremities. Routine laboratory studies, heavy metal screen, vitamin, cryoglobulin, coproporphyrin and delta-amino levulinic acid in urine, and the protein value of the cerebrospinal fluid were normal. Head and neck MRI, and myelography were normal. Immunofixation electrophoresis showed IgM lambda M-protein in serum. Thin-layer chromatography with immunostaining showed his serum IgM reacted with GM1, GD1b, and asialo GM1. ELISA (Enzyme Linked Immunosorbent Assay) demonstrated high titers of anti GM1, GD1b and low titer of anti asialo GM1. Motor conduction studies showed no demonstrable conduction block, normal conduction velocities and the low amplitudes of CMAP. Sensory conduction studies showed no abnormalities except for slightly decreased amplitude of SNAP in sural nerve. Electromyography showed active denervation in extensor digitorum communis muscle, tibialis anterior muscle and left biceps brachii muscle. Muscle biopsy specimen revealed large and small group atrophy and there was perivascular mononuclear infiltration at one point.(ABSTRACT TRUNCATED AT 250 WORDS)