Diabetes mellitus is the most common cause of the peripheral neuropathy and an important risk factor of the cerebrovascular disease. There are also several case reports of the diabetic myelopathy, and it has been known that the incidence of cord involvement is not rare on autopsy and somatosensory evoked potential studies. To determine the functional derangement of central descending motor pathway in diabetes, 59 cases of diabetes were studied with the magnetic motor evoked potential (MMEP) study. The diabetic patients were divided into two groups according to the nerve conduction study (23 with normal and 36 with abnormal nerve conduction studies) and compared with the results of 31 normal control cases. The results are as follows. 1. At each stimulated point, there are statistically significant differences in latencies of MMEPs between normal control and diabetic groups. 2. Compared to the control group, the peripheral conduction time is also significantly prolonged in diabetic patients. 3. However, there is no difference in the calculated central motor conduction time among three groups. In conclusion, this result means that prolonged latencies of MMEPs in diabetes may be due to peripheral neuropathy rather than dysfunction of central motor pathway. Therefore the clinical significance of MMEP has to be decided segmentally rather than by simple determination of the vertex latency.