14 patients with tumors in generalised stages (Hodgkin's disease, oat-cell-carcinoma, Fibrosarcoma, acute leukemia) were treated altogether 54 times with cytostatics in combination with hyperthermia. The temperature was induced by microwaves with a frequency of 27 MHz. A temperature of 40 degrees C was reached after 40 min. At this point cytostatics were applicated; afterwards the temperature was maintained for one hour at 40-40,5 degrees C. Cardia, pulmonary and circulatory complications did not occur. Controls of the laboratory parameters could exclude effects on electrolytes, muscles, blood, liver and kidney. The laboratory controls were made before, during, immediately after and 24 h after hyperthermia. There were no signs for an enhancement of toxicity typical for cytostatics. The observations are compared to the results of other investigators. To date the therapeutic effect of this treatment can not be stated.