Induction of hyperthermia by ultrasound is facilitated by the characteristics of sound absorption in tissue: at frequency of 500 kHz the penetration distance in muscle is about 12.5 cm. Energy deposition patterns can be shaped by focussing or by the use of multiple transducers. An initial clinical study has demonstrated the safety and efficacy of the heating technique. Temperatures of 43-45 degrees for 30-45 min were obtained in surface accessible tumors. 50 per cent of the treated tumors regressed either partially or totally. Normal tissue damage was minimal even in sites that had previously been heavily irradiated. A few patients given relatively small doses of X-irradiation in addition to the hyperthermia responded very well. Based upon these encouraging results, a new ultrasound system designed for heating of deep seated lesions has been built and tested.