A percutaneous, low power, interstitial method of controlled tissue coagulation by laser light has been developed and assessed as a possible alternative to existing therapy for benign and malignant prostatic disease. For prostate cancer this technique may be of importance when the tumor volume is small and well defined, particularly as these tumors can increasingly be identified by transrectal ultrasound (TRUS) and are unlikely to have metastasised. The possibility therefore arises of destruction of such small lesions in situ, thereby avoiding the need for radical surgery or radiotherapy. Using the male beagle prostate model, one or more 150-400 micron fibers was implanted within the substance of the prostate through which a Yttrium Aluminum Garnet (YAG) laser energy could be transmitted. Using long exposures and lower powers than used in routine endoscopic laser therapy, well-defined areas of coagulative necrosis could be created without extensive tissue charring or damage to the fiber. For an energy dose of 1000J a lesion approximately 1 cm in diameter results at four days. The ultrasound scanning methods can detect the fiber(s) and the area(s) of coagulation. At 6/52 months following treatment, healing was by fibrosis or cystic degeneration. There were no ill effects on the subjects following coagulation of the prostate. Multiple fiber experiments produced larger volume lesions relevant to more extensive cancer or for the coagulation of benign adenomatous hyperplasia causing outflow symptoms. Ultrasound guided clinical treatments in patients with cutaneous metastases or hepatic and pancreatic tumors have confirmed the experimental results so far obtained. The technique may prove of value for the destruction of early, small, focal tumors of the prostate and for the treatment of moderate benign enlargement. Clinical trial for both benign and malignant disease has commenced.