Changes in median nerve somatosensory evoked potential (SEP) in diabetic patients have amply been reported. However the relationships between the diabetic polyneuropathy and the abnormalities of SEPs are still a matter of discussion. To elucidate these relationships, we have studied SEPs in 38 non-insulin-dependent diabetics with mild or severe degree of peripheral neuropathy. Our principal findings are as follows. (i) The wrist-Erb point (EP) conduction velocity (WECV) and the EP-N13 interpeak latency are significantly altered in diabetics. The N11-N13, N13-N20 and N13-P22, interpeak latencies are within the normal range. (ii) The WECV is more reduced in patients with severe neuropathy than in those with mild neuropathy, whereas the degrees of neuropathy have no influence on the alterations of EP-N13 interpeak latency. (iii) No significant correlation is found between the WECV and the EP-N13 interpeak latency. (iv) The WE conduction velocity was significantly correlated with both motor and sensory peripheral nerve conduction velocities. On the contrary, no correlation is found between these latter peripheral parameters and the EP-N13 interpeak latency. These findings indicate that the alteration in the tract from brachial plexus to the posterior column could not be a parallel process of distal neuropathy of non-insulin-dependent diabetic patients.