To examine the results of thermoregulatory sweat testing in patients with erythromelalgia and to compare them with the results of other neurophysiologic tests of small-fiber nerve function. Retrospective study. Tertiary referral center. Thirty-two consecutive patients with erythromelalgia who had thermoregulatory sweat testing in addition to vascular and nerve testing. The following information was abstracted for each patient: demographics, clinical presentation, and results of thermoregulatory sweat testing, vascular (noninvasive) testing, and nerve testing (electromyography and autonomic reflex screen, including quantitative sudomotor axon reflex test). Results of thermoregulatory sweat testing to evaluate small-fiber neuropathy, compared with other tools used to estimate small-fiber neuropathy. Thermoregulatory sweat testing results were abnormal in 28 (88%) of 32 patients, and quantitative sudomotor axon reflex test results were abnormal in 22 patients (69%). Abnormalities noted on thermoregulatory sweat testing varied from local hypohidrosis or anhidrosis to global anhidrosis. Global or almost-global anhidrosis was present in 8 patients (25%); in 19 patients (59%) the anhidrosis was distal, and 1 other patient (3%) had a less specific pattern of anhidrosis (multifocal or regional). The area of anhidrosis generally corresponded to the area that was symptomatic of the erythromelalgia. Small-fiber neuropathy is prevalent in most patients with erythromelalgia. Thermoregulatory sweat testing is a sensitive and useful marker of small-fiber neuropathy in these patients.