Objective. Diabetes mellitus is becoming a major cause of premature disability in Japan, and peripheral neuropathy is a common complication of diabetes. The aim of this study was to evaluate the relationship between the results of nerve conduction studies (NCS) and the size of the nerve determined by sonography in diabetic patients. Methods. Twenty diabetic patients (mean age ± SD, 57.1 ± 13.6 years) and 20 healthy volunteers (mean, 61.1 ± 8.9 years) were enrolled in this study. Patients' wrists that had symptoms of carpal tunnel syndrome were not included in the study; those that were included had negative Phalen test results. We then divided the patients into 2 groups (patients with and without diabetic symmetric polyneuropathy [DPN]). The cross‐sectional area (CSA) was measured in the carpal tunnel 5 cm proximal to the wrist and elbow joint of the median nerve. Results. There was a significant increase in the CSA in patients with DPN in the carpal tunnel compared with the control participants (P < .01) and patients without DPN (P < .01). The CSA in the carpal tunnel showed a significant correlation with the motor nerve conduction velocity (r = −0.473). Conclusions. The CSA of the median nerve in the carpal tunnel of patients with DPN is greater than that in patients without DPN and healthy individuals and correlates with NCS.