To investigate the effectiveness of using the cross-sectional area of the ulnar nerve on the asymptomatic side as a reference value to assess changes in the thickness of the affected ulnar nerve in unilateral ulnar neuropathy patients. Methods: A total of 47 patients with unilateral ulnar neuropathy and 60 normal volunteers were recruited to the study as the observation group and control group, respectively. High-frequency ultrasound was used to measure the cross-sectional area of the ulnar nerve, and the differences between the asymptomatic side in the observation group and the control group was compared. After the observation group was cured, the cross-sectional area of the asymptomatic side ulnar nerve was reexamined by ultrasound and compared with the control group for significant difference. Results: There were statistically significant differences in the cross-sectional area of the ulnar nerve at four sites on the asymptomatic side of the observation group compared with the control group (P<0.05). After patients were cured, there was no significant difference in the cross-sectional area of the asymptomatic side ulnar nerve between the observation group and the control group (P>0.05). Conclusion: The asymptomatic side of patients with unilateral ulnar neuropathy might also have lesions to some degree. The use of the cross-sectional area of the asymptomatic side ulnar nerve as a reference value to assess ulnar neuropathy on the affected side is limited.