Urinary outflow obstruction by prostatic enlargement is usually treated by resection or, recently, less invasively by thermal `ablation' of tissue through the urethra. With the latter technique, the amount of tissue that can be removed is limited by the limited penetration depth of suitable radiation sources, e.g. lasers, or conduction of heat. Interstitial thermotherapy was expected to overcome this problem. Our initial in vitro and animal studies with different light guides for interstitial application of Nd:YAG laser radiation showed small carbonized lesions with bare fibers, but large homogeneous coagulation zones with special `ITT' (interstitial thermotherapy) fibers. Further studies using these applicators resulted in a technique to be apt for clinical routine in the treatment of symptomatic prostatic enlargement. The tip of the light guide was repeatedly inserted into the prostate either transurethrally through a cystoscope under direct vision or percutaneously from the perineum under transrectal ultrasound guidance. The number of fiber placements depended on the size and configuration of the gland. Irradiation was performed either for 10 min with 5 or 7 W or in the advanced `turbo'- mode for 5 or 3 min per fiber placement using automatically stepwise reduced power (20 W for 30 s, 15 W for 30 s, 10 W for 30 s, and 7 W for 210 or 90 s). By optical feedback control the laser was switched off automatically in the case of carbonization to avoid fiber damage. From July 15, 1991 to October 1, 1993 239 patients with BPH and 14 patients with advanced prostate cancer, suffering from severe urinary outflow obstruction, were treated by laser induced interstitial thermotherapy. The results and complications of treatment are reported.