Because most of the diseases to which radon (222Rn) therapy is applied are related to activated oxygen, in this study the effect of the radioactivity of radon and the thermal effect were compared under a room or a hot spring condition with the same chemical component using as the parameters the activity of superoxide dismutase (SOD), which is an oxidation inhibitor, and lipid peroxide and low density lipoprotein (LDL)-cholesterol, which are closely involved in arteriosclerosis. Results show that the SOD activity was significantly increased, and the lipid peroxide and LDL-cholesterol levels were significantly decreased on days 6 and 7 of study. The results were about 2-fold larger in the radon group than in the thermo group. This suggests that the anti-oxidation function was more enhanced by radon therapy than by thermo therapy, and suggests that radon therapy may help to prevent the causes of life style-related diseases such as arteriosclerosis. These findings are important in understanding the mechanism of diseases in which radon therapy is used as treatment, and most of which are called activated oxygen-related diseases.