The motor system of 13 cases with hereditary motor sensory neuropathy (HMSN) type I were analysed by clinical neurophysiological method. The motor conduction velocity (MCV) and F wave latency in lower motor neurone were markedly delayed. The latency of the muscle evoked potential (MEP) by cortical magnetic stimulation were also markedly delayed. The central motor conduction times (CMCT) were calculated by two methods. CMCT-mag was calculated by subtraction of the MEP by cervical magnetic stimulation from the MEP by cortical magnetic stimulation. CMCT-f was calculated by subtraction of the [(F wave latency -1 + distal latency)/2] from the MEP by cortical magnetic stimulation. There were positive correlation between CMCT-f and CMCT-mag. CMCT of HMSN type I were divided to two groups. CMCT of the first group was markedly delayed. CMCT of the second group was mildly delayed or normal. The former group showed marked weakness in distal muscles clinically. The latter group showed mild or moderate weakness in distal muscles clinically. All these patients did not show any pyramidal tract signs, which could be covered by severe lower motor neurone involvements. The classification of HMSN type I by gene was well known, genetical analysis might be important to these groups in HMSN type I.