Objective: Diabetic autonomic neuropathy is a common, serious and even fatal complication. The aim of this study was to investigate the autonomic functions in diabetic patients with distal symmetric polyneuropathy, via clinical and electrophysiological tests. Material and Methods: This study was performed on 50 patients with diabetic distal symmetric neuropathy (patient group) and 30 healthy volunteer subjects (control group). To assess autonomic functions, sympathetic skin response (SSR) and RR interval variability (RRIV) were studied, and the corrected QT interval (QTc) and QT dispersion (QTcd) were calculated with electrocardiography (ECG) in both patient and control groups. Data obtained from the control and the patient groups were compared. The sensitivity and specificity of the autonomic tests were measured. Results: In 98% of 44 patients, foot or both foot and hand SSR could not be registered. Survey of autonomic symptoms (SAS) scores were higher in the patient group in whom SSR could not be recorded when compared to the patient group in whom SSR could be recorded. Almost all the parameters of RRIV (RR interval variation) were significantly lower in the patient group when compared to the control group (p<0.001) The highest sensitivity and specificity among all RRIV parameters belonged to RRIVD and RRIVD- RRIVR. Highly statistically significant (p<0.001) increases in QTc and QTcd values were detected in the patient group in comparison with the control group. The specificity and sensitivity rates of both QTc and QTcd were higher. In addition, significant correlations were found between both RRIV and QTc/QTcd, and the scores of SAS. Conclusion: Our results have shown that SSR, RRIV and QTc-QTcd are very valuable electrophysiological tests for evaluation of autonomic functions in diabetes mellitus patients. Our study also showed that application of these tests together would be more useful. © 2014 by Türkiye Klinikleri.