We present a case with stepwise weakness and sensory involvement of both hands for more than 2 months. His nerve conduction study findings revealed prolonged terminal latencies, decreased motor and sensory conduction velocities and conduction blocks of both ulnar nerves, more severely on left side. And there were other abnormalities manifested with mononeuropathy multiplex. Increased cerebrospinal fluid protein was found. We diagnosed him as Lewis-Sumner syndrome and tried high dose oral steroid therapy for 2 months. He showed improvement of motor functioning with persistent conduction block.