We report a case of O'Sullivan-McLeod syndrome in a 59-year-old man, who had experienced slowly progressive weakening of both hands since he was 20 years of age. Mild hyperIgEemia and eosinophilia were present. Nerve conduction studies revealed reduced F wave-evoked frequencies for the median and ulnar nerves. Intravenous immunoglobulin (IVIG) at a dose of 400 mg/kg/day was given for 5 days. After IVIG, the muscle weakness of the distal upper extremities improved together with increased F wave-evoked frequencies. These effects lasted for a few months. These observations suggest that immune-mediated neural damage partially contributes to O'Sullivan-McLeod syndrome.