Conditions described as "erythromelalgia" and "erythermalgia" are being formally specified by etiological diagnoses that enable the use of disease-modifying as well as symptomatic treatments. We describe an otherwise healthy 20-year-old man with acute-onset erythromelalgia. Severe bilateral distal limb pain and vasodilation persisted despite the use of many antihyperalgesics. Pathological examination of cutaneous nerve endings revealed severe small-fiber predominant axonopathy. Treatment of his apparent autoimmune polyneuropathy with high dose corticosteroids, 4 days of lidocaine infusion, and a prednisone taper cured him. Similarities to other cases allowed us to tentatively characterize a new treatable cause of erythromelalgia; acute adolescent autoimmune small-fiber axonopathy. In this report we evaluate various options for diagnosis and treatment.