The current study outlines the historic development and crystallization of nomenclature of a syndrome of red, warm, swollen, and painful extremities. This syndrome can he divided into three types: erythromelalgia, primary erythermalgia, and secondary erythermalgia. We have recognized that a subset of patients who experienced clinical relief with aspirin had elevated platelet counts either associated with primary thrombocythemia or thrombocythemia associated with another chronic myeloproliferative disorder, polycythemia vera in particular. This form is defined as a distinct clinical entity and depicted as erythromelalgia. On the basis of 10 characteristics, erythromelalgia is set apart from other forms of red, warm, and painful extremities in the absence of thrombocythemia, which we labeled erythermalgia. Erythermalgia occurs in two different forms. Primary erythermalgia appears to arise at a young age as a bilateral symmetric burning and redness of mainly the lower extremities in the absence of detectable disease. Secondary erythermalgia is typified by red, warm, swollen, and painful extremities in the absence of thrombocythemia. This entity is linked to other (vascular) illnesses or the use of drugs. Secondary erythermalgia usually responds to treatment of the underlying disease or stoppage of the incriminated drug.