CO2 baths are considered an integral feature of balneotherapy. Natural springs containing a concentration of CO2 probably sufficient to be clinically effective (> 1 g/l) as well as artificially enriched baths are applied. Phenomena like a marked erythema (at concentrations above 0.5 g/l) imply clinical efficacy beyond unspecific effects of baths (water immersion, thermal effects). Prolonged effects are postulated to exceed "direct" effects, thus forming the rationale for serial applications of CO2-baths. CO2 is believed to cause an acute induction of local vasodilation and a shift of the O2 binding curve, resulting in a facilitation of the delivery of O2 to the tissue. A positive impact of CO2-baths on the flow properties of blood has been reported. A wide variety of indications is mentioned in the literature, while clear evidence from controlled trials exists only for a minority of them, mainly for chronic circulatory disturbances based on atherosclerotic diseases such as peripheral arterial occlusive disease, trophic ulceration, microangiopathies of various origins, and mild hypertension. Some evidence supports the idea that CO2 baths might represent an efficient therapeutic means in the rehabilitation of coronary heart disease, myocardial infarction and stroke, and in the treatment of chronic venous insufficiency, certain inflammatory diseases, and functional disturbances.