Hirayama disease is characterized by focal amyotrophy of the upper limb associated with unilateral or asymmetric bilateral muscle weakness. It occurs in young men in their late teens or early twenties. Repeated or sustained neck flexion is thought to be a cause of the disease. It is important to prevent the progression of the disease through early recognition and management. We reported a patient with Hirayama disease in order to review the typical clinical, electrodiagnostic and radiological findings of Hirayama disease. (J Korean Acad Rehab Med 2004; 28: 387-391)