We previously reported that decrement of compound muscle action potential (CMAP) by repetitive nerve stimulation (RNS) was greater in the median nerves than in the ulnar nerves of patients with amyotrophic lateral sclerosis (ALS). The aim of this study was to evaluate whether CMAP decrement by RNS is a feasible marker for the differentiation of ALS from other diseases.We performed RNS in the median and ulnar nerves of 51 patients with ALS and 40 patients with other diseases.The CMAP decrement was significantly greater in the median nerves of patients with ALS, compared to the disease control patients. In the median nerves of patients with ALS, CMAP decrement was significantly greater in the cervical region-onset group than in the other region-onset group.The finding of CMAP decrement in the median nerves can be useful for differentiating ALS patients with cervical region onset from other controls with active neuropathic diseases.