Several methods to administer systemic hyperthermia (41.8-42.0 degrees C) as an oncologic procedure have been described in the literature. We have developed a method to induce and maintain a body temperature of 42.0 degrees C (rectal) using a high-flow water immersion bath. This method was tested in a small series of sheep and proved to be applicable. In a preliminary clinical series of 10 treatment sessions advantages compared to the described other methods were noted. After a short induction period, accurate temperature control can be maintained. During the procedure the patient can be observed and handled, while buoyancy in water is an asset. Short term clinical were favourable.