A joint clinical trial of hyperthermia using a newly developed 8-MHz radiofrequency (RF) capacitive heating device (Thermotron RF-8; developed in cooperation with Yamamoto Vinyter Co. of Osaka) was performed under collaboration of seven institutions. Radiation with 4 Gy twice a week for a total of 40 Gy or 2 Gy five times a week for a total of 50 Gy was delivered. After irradiation, hyperthermia at 42.5 degrees C +/- 0.5 degree C for 40 to 60 minutes was given twice a week for a total of 10 times. Tumors examined in this trial were located in various depths in the body, and included those which were considered refractory to conventional treatments or radioresistant such as malignant melanoma and soft tissue tumors. Of the 63 tumors treated, 52.4% showed complete regression (CR); 19.0% more than 80% regression (PRa); 20.6%, 80% to 50% regression (PRb); and 8.0% no regression (NR). Our joint clinical trial demonstrated that hyperthermia with the use of the Thermotron RF-8 is safe and effective in the treatment of radioresistant tumors located in superficial, subsurface, and in some cases deep regions, if the surface cooling is properly managed by the temperature-controlled saline pad and electrodes of appropriate size are paired.