Objective: To evaluate whether sympathetic skin response (SSR) differs in patients with hereditary sensory autonomic neuropathy (HSAN) types III and IV. Background: HSAN types III and IV are rare autosomal recessive disorders that cause many similar autonomic, sensory, and motor dysfunctions, but different sweating characteristics. HSAN III patients have preserved and at times, excessive sweating, whereas anhidrosis is characteristic of HSAN IV. SSR reflects the integrity of sympathetic sudomotor fibers and the activation of sweat glands through the change in skin resistance in response to an arousal stimulus. Therefore, SSR is a test method that might facilitate differential diagnosis of HSAN III and IV. Methods: In 17 HSAN III patients (eight women, nine men; mean age, 20.65 ± 5.45 years) arid seven HSAN IV patients (five girls, two boys; mean age, 10.0 ± 5.45 years) SSR was recorded from the palms and soles after repeated electrical, acoustic, and inspiratory gasp stimulations. In addition, all subjects underwent a neurologic examination; studies of median, peroneal motor, and sural nerve conduction velocities; and determination of vibratory and thermal perception thresholds. Results: Although clinical differences were appreciated between the two types of HSANs, both HSANs had evidence of small-fiber involvement. Both HSANs had abnormal temperature and pain perception. In contrast, SSR was preserved in all HSAN III and absent in all HSAN IV patients. Conclusion: SSR provides another parameter to improve differentiation of HSAN III from HSAN IV, and also gives us additional information regarding sympathetic sudomotor fiber function in these developmental diseases.