From the beginning of radiotherapy, efforts have been made to improve the therapeutic effects X-rays by heat. Recently, the combined therapy of heat and X-ray therapy has gained new interest because of failures of the megavoltage therapy of solid malignant tumors. Encouraged by our own animal experiments with a Walker-tumor of rats, since 1972 we have treated 52 patients with tumors of lower sensitivity. We used decimeter waves; wave length of 69 cm and a frequency of 433.92 MHz. After preheating for 3 to 5 min we added the gamma-rays of Caesium-137 or Cobalt-60 in usual rhythms. The total dose only in few cases exceeded 6000 to 6500 rads in six to seven weeks. We observed a relatively very good response for tumors of low sensitivity. The dose was between 20 and 30% lower which in the treatment of tumors of median sensitivity is acceptable. Local recurrences are rare and occurrence of distant metastases are within the characteristic development of certain malignant tumors. A prospective study has begun.