Hyperthermia - the sustained controlled heating of tissues to 42-43 degrees C - is being studied and used as a means of treatment, particularly to selectively destroy malignant cells. This may be achieved by heat alone, or in conjunction with other forms of treatment, such as radiotherapy or chemotherapy, thanks to a synergistic effect obtained. A new microwave apparatus has been designed and developed which permits precise, controlled and safe hyperthermic treatment of the human prostate. Following extensive laboratory and experimental studies, a first series of treatments on humans has been carried out. Initial observations on our first 192 treatments, on 32 patients, are reported.