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This study aimed to evaluate sympathetic skin response (SSR) among patients with Raynaud phenomenon (RP). SSR is a technique for assessment of the damage of peripheral neuropathies and the disorders of the sympathetic system. Methods: Between January 2015 and December 2018, approximately 20 patients with RP and 20 healthy subjects (control group) were recruited from patients referred to the outpatient clinics of Shiraz University of Medical Sciences, Shiraz, Iran. All participants were clinically examined and the SSR was evaluated using a standard protocol. SSR is abnormal when the latency is prolonged and/or the amplitude is reduced. Results: The RP group consisted of 19 women (95%) and one male (5%); three patients (15%) had primary Raynaud's phenomenon (PRP) and 17 patients (85%) had secondary Raynaud's phenomenon. The control group consisted of 16 women (80%) and four males (20%). The mean age of the RP group and control subjects was 43.1 ± 9 and 36.7 ± 8.6 years, respectively. The SSR to the electrical stimulus was absent in three patients with PRP. The total median nerve mean latencies in the upper limb were 1.90 ± 0.57 and 1.19 ± 0.52 seconds for the RP group and control groups, respectively (P <0.001). These findings revealed significantly prolonged SSR latencies in the RP group, while the mean amplitude showed no significant differences in both groups (P = 0.756). Conclusion: Absence or prolonged latency of SSR was associated with the disorders of the unmyelinated axons in the sympathetic system. The findings of the present study suggested the disorders of unmyelinated axons in Raynaud's phenomenon. [ABSTRACT FROM AUTHOR]