An apparatus and method for the treatment of diseases of the prostate, including BPH and prostate cancer, utilizing transurethrally applied focussed ultrasound energy to produce hyperthermal and thermotherapeutic effects in diseased tissue are disclosed. A first preferred embodiment of the apparatus relies on visual control; and a second preferred embodiment of the apparatus incorporates an integrated ultrasonic imaging and therapy transducer. The apparatus and method feature the close proximal placement of the ultrasound transducer to the diseased tissue to be treated, resulting in less attenuation of the beam and deposition of a greater fraction of the total emitted ultrasound energy in the selected portion of the diseased tissue. The ultrasound energy passes atraumatically through the urethral wall and other tissue not targeted. At the focal point of the ultrasound energy, substantial heating of diseased tissue to produce hyperthermal effects therein occurs, and even higher thermotherapeutic temperatures are attainable to produce coagulative necrosis of diseased tissue.