An ultrasound unit for the clinical application of hyperthermia to tumor cells in soft tissue. The ultrasound hyperthermic unit includes in one aspect an array of sonic transducers (1) radially disposed about an axis and angled to direct sonic energy (12) towards said axis at a point remote from said array, each transducer being adapted for angular movement relative to the other transducers thereby to allow adjustment of the shape of the acoustic focus. The array is computer controlled both with regard to acoustic focus and with regard to the energy imparted by each member of the array, said control being mediated by temperatures sensing means (40) adjacent a tumor. This ultrasound unit may be associated with a heat treatment tank (51, 31) including a flexible membrane (32, 52) which may overlie a tumor, thereby to locally raise the temperature and reduce the amount of ultrasonic power required to achieve a desired temperature. Suitable adjustment of the focused energy raises the temperature of the tumor cells above viability whereas the same temperature applied to healthy cells still allows viability by virtue of their relatively greater heat dissipation ability.