Background Hirayama disease is a rare disease characterized by juvenile-onset of asymmetric amyotrophy,of which etiology has not been clarified.The aim of our study was to investigate the clinical and neurophysiologic characteristics of Hirayama disease.Methods Neurophysiological tests,including nerve conduction studies (NCS),F-wave and routine electromyography (EMG),were performed in seventy-three patients with Hirayama disease.EMG was selectively performed on upper and lower extremities,sternocleidomast and thoracic paravertebral muscles according to the clinical features of the patients.Results Abnormal NCS parameters,including decreased compound muscle action potentials or delayed distal motor latency,were found in 34.2％ (25/73) and 12.3％ (9/73) of the patients,respectively.A total of 24.6％ (18/73) of the patients showed decreased F-wave frequency.EMG demonstrated the presence of neurogenic lesions in all patients with spontaneous potentials,prolonged duration or augmentation of amplitude in motor unit potentials (MUPs),or a single pattem of MUP recruitment.About 17.8％ (13/73) of the patients showed neurogenic lesions,mostly in the C7-8 level of the cervical cord,only in the upper extremity of affected side,whereas 35.6％ (26/73) of the patients possessed lesions in the upper extremities bilaterally.A total of 46.6％ (34/73) of patients exhibited abnormalities in the lower extremities,stemocleidomast or thoracic paravertebral muscle.Changes in motor NCS were significantly correlated with muscle strength.Conclusions EMG detects diffused subclinical neurogenic lesion in a high proportion of patients with Hirayama disease.Results of our study challenge the hypothesis that Hirayama disease is a type of cervical myelopathy.