The response of a mouse's foot to heat was studied. Transplanted syngeneic tumor, C3H mouse mammary carcinoma, was treated with irradiation and hyperthermia in a waterbath. The tumor did not disappear in any of the mice treated with radiotherapy with a dose of 20 Gy alone, but disappearance of the tumor was observed in 11 of 15 and 6 of 8 of the mice treated with combined therapy of irradiation and hyperthermia. There was a significant difference between these two groups. Synergistic effect was confirmed (P less than 0.001, P less than 0.005). Hyperthermia using Thermotron RF-8 was performed on 19 patients (5 bladder cancers, 3 uterine cancers, 3 rectal cancers, 4 soft tissue tumors, 2 oral cancers, 1 biliary tract cancer, 1 renal cancer) between April, 1986 and December, 1986. They were irradiated with a daily dose of 1.5-2.0 Gy, 5 times a week and hyperthermia was performed within 30 minutes after each irradiation once or twice a week. Intratumoral temperature was kept at 43 degrees C-45 degrees C. Temperature over 41 degrees C was maintained in most patients. Clinical response was assessed by tumor regression rates. Partial response a (PRa), defined as 80% or more regression in tumor volume, was obtained in 1 bladder cancer patient and PRb, defined as 50% to less than 80% regression, was obtained in another 5 patients. Side effects were observed in all patients including mild skin burn, nausea and diarrhea. Rectovaginal fistula developed in 1 patient. Combined radiotherapy and hyperthermia seems to be useful in advanced cancer patients.