A case of early-onset and slowly progressive chronic inflammatory demyelinating polyneuropathy (CIDP) was reported. Her progressive gait disturbance began at six years of age and she developed pes cavus. At the age of 13 years, a diagnosis of CIDP was made, and oral corticosteroid therapy was started. This therapy was effective, but the disease relapsed soon. Muscular strength improved after supplementation of an immunosuppressant with oral corticosteroid, following steroid pulse therapy. On peripheral motor conduction study, M waves showed very low amplitudes and remarkably delayed onset-latencies. New units of M wave appeared and amplitudes increased soon after initiation of the corticosteroid therapy because of an improvement of conduction block, and durations of M wave were prolonged. Then, in accordance with shortening of M wave latencies, some dispersed units were synchronized and durations reduced. At relapse, changes of M wave showed an inverse relationship to the changes of M wave with clinical improvement. The electrophysiological findings started improving 1 week after the therapy, while clinical improvement was detected several weeks later. We conclude that the change of M wave was a sensitive marker to evaluate the effect of the therapy in our CIDP case.