A 31-year-old man had noticed slowly progressive weakness in his right upper limb girdle. On admission at age 32, fasciculation and muscle atrophy were observed in the right pectoralis major muscle. There was mild muscle weakness in the right triceps brachii muscle. The deep tendon reflexes were normal and the pathologic reflexes were absent. Sensory and autonomic nerve functions were intact. Needle EMG showed fibrillation potentials and positive sharp waves in the right pectoralis major, infraspinatus, extensor carpi radialis, extensor digitorum muscles, the bilateral sternocleidomastoideus and triceps brachii muscles. Motor nerve conduction studies revealed normal conduction velocities and distal latencies as well as no evidence of conduction block or abnormal temporal dispersion. Serum immunoelectrophoresis failed to detect an M protein. Thin-layer chromatography with immunostaining revealed that the serum contained auto antibodies which reacted with asialo-GM1, GM1 and LM1. This case must belong to "proximal lower motor neuron syndrome" proposed by Pestronk et al.