The diagnosis of erythermalgia, initially made in 27 patients between 1980 and 1986, was re-evaluated on the basis of 7 criteria. Three were major criteria: paroxysmal attacks, burning pain in the extremities and redness of the territory concerned during the attacks. The 4 minor criteria were: typical precipitating factors (exposure to heat, effort), typical relieving factors (exposure to cold, rest), elevated local temperature during the attacks and response of symptoms to acetylsalicylic acid. The diagnosis was deemed to be correct when the 3 major criteria and at least 2 of the minor criteria were present. Thirteen patients (8 women, 5 men) fulfilled these conditions. Nine of them had primary erythermalgia and in 4 patients the condition was consecutive to a myeloproliferative syndrome (thrombocytopenia in 2 cases, Vaquez' disease in 2 cases). These two forms differed on several points. Patients with secondary erythermalgia were older, some had unilateral disorders, and their symptoms were less intense; the syndrome always followed a favourable course and disappeared when the causative disease was cured; in 3 out of 4 cases erythermalgia disclosed a myeloproliferative syndrome. Patients with primary erythermalgia were younger, the syndrome was of longer duration and the symptoms always bilateral and sometimes severe; those who responded to acetylsalicylic acid had a favourable prognosis, but treatment was difficult in the others. Capillaroscopy is of little help to diagnose this syndrome; this is done on clinical grounds only and it is easy when the criteria, as defined in this study, are present. In every case, blood examination with platelet count and erythrocyte sedimentation rate is advisable.