We investigated 303 diabetic patients in order to clarify the relationship between progression of diabetic polyneuropathy and conduction delay across the carpal tunnel. Distal latency ratio (DLR) was determined by comparison of distal motor latency of the median nerve with that of the ulnar nerve. Lower extremity polyneuropathy index (LPNI), expressed as a mean percentage of the normal for six indices over two nerves obtained by motor nerve conduction studies, was 82.9% on the average in the patients. Their DLR (1.44 +/- 0.24) was larger than the normal value (1.29 +/- 0.10). About 30% of the diabetics had abnormal DLR, especially in women its incidence was as high as 39%. The lower the LPNI level, the larger the incidence of abnormal DLR. In diabetic polyneuropathy patients peripheral nerves will become fragile, which might increase the incidence of conduction delay across the carpal tunnel. This phenomenon might also be called as 'double crush syndrome'.