The effect of repetitive mild hyperthermia on body temperature, the autonomic nervous system, and innate and adaptive immunity was investigated using a new hyperthermia treatment system, nanomist sauna (NMS). Six healthy volunteers participated and the concentration of catecholamines and cortisol, and the frequency and function of leukocytes in the peripheral blood were investigated before and after successive 7 days of hyperthermia treatment (20 min/day, 40 degrees C, 100% relative humidity). After treatment, the blood level of adrenaline and cortisol on the 7th day was decreased compared with the 1st day, indicating the suppression of the sympathetic nervous system activity. Moreover, the frequency of CD56(+)NK, CD56(+)NKT and B cells on the 7th day tended to be increased compared with the 1st day. The frequency of HLA-DR-positive NK and NKT cells and expression of HLA-DR on B and T cells increased. The cytotoxicity of NK cells and proliferative response of B cells were also elevated. The results indicate that repetitive mild hyperthermia treatment might suppress excessive sympathetic dominance and modify immunity. Additionally, because it can provide the same effects as conventional hyperthermia treatments with minimal burden to the body, NMS may be a novel patient- and elderly-friendly hyperthermia treatment for health promotion.