We describe a percutaneous, low power, interstitial method of controlled coagulation of prostatic tissue by laser light which may prove an alternative or adjunct to existing therapy for benign and malignant prostatic disease. One or more 200-600 micron diameter fibres were implanted within the substance of the elderly male beagle prostate (n = 11) through which Neodymium-Yttrium Aluminium Garnet (Nd-YAG) laser energy could be transmitted. Using longer exposures (400-1500s) and lower powers (1-2W) than used in routine endoscopic laser therapy, well defined areas of coagulative necrosis could be created with little tissue charring or damage to the fibre. For an energy dose of 1000J a lesion approximately 1 cm in diameter resulted at 4 days. Ultrasound scanning methods could detect the fibre(s), the area(s) of coagulation and the healed lesions. Treatments were well tolerated. At 6 weeks the treated areas of canine prostate healed by fibrosis surrounding an area of cystic degeneration. Multiple fibre experiments produced larger volume lesions relevant to more extensive cancer or to the coagulation of benign adenomatous hyperplasia causing outflow symptoms. This technique may prove of value for the treatment of moderate benign enlargement of the prostate and for the destruction of small, focal prostatic tumours.