Therapy using radon or thoron gas is performed for various diseases such as diabetes mellitus and rheumatoid arthritis. The aim of this study is to analyze the effects of thoron and thermal treatment for life- style-diseases on humans. The subjects were 76 persons who divided into 3 groups: normal, diabetes mellitus and rheumatoid arthritis. The all subjects were inhaled thoron in a Hanamaki spa (Iwate, Japan) with a high concentration (about 4900 Bq/m³) for 2 weeks. The levels of free fatty acid, immunoreactive insulin and ketone bodies of diabetes mellitus group came to the normal levels by thoron and thermal treatment. The α-atrial natriuretic peptide level of rheumatoid arthritis group was increased and the blood pressure was significantly decreased. Superoxide dismutase activity of rheumatoid arthritis group was significantly increased by treatment. In addition, thoron and thermal treatment significantly enhanced concanavalin A-induce mitogen response and increased the level of CD4 positive cells; it decreased the level of CD8 positive cells. The results suggest that thoron and thermal treatment activates antioxidative function. Furthermore, these findings suggest the possibility that the thoron and thermal treatment prevents diabetic ketoacidosis, and, contributes to the prevention of life-style-disease, which relate to peroxidation reactions and immune suppression, by an enhancement of the immunity function. These indicate that may be a part of the mechanism for the alleviation of diabetes mellitus and rheumatoid arthritis brought about more by thoron and thermal therapy.