Each year, Royat (Auvergne) receives about 20,000 patients (80 per cent with intermittent claudication), treated by thermal gas (99.5 per cent of CO2). CO2 therapy is dispensed with thermal water or with dry gas (general or local immersion and local subcutaneous injections of gas) during 3 weeks. Local vasodilator effects of CO2 have been demonstrated with several methods in Royat. Physiological and therapeutic effects of thermal CO2 therapy, also used in Germany and Central Europe, were precisely reported during the Consensus Congress of Fribourg in Brisgau (1989); more particularly, this treatment seems provide a reduced ability of haemoglobin to fix oxygen, and therefore a release of oxygen within the cells. Patients with peripheral arterial disease (stage 2) have a walking distance increased and post-exercise ankles' pressures improved after a thermal course of treatment in Royat, while a control group has no significant changes. The discussion will concern also the socio-economic aspects of thermal treatment of arterial diseases.