Objectives: To evaluate the safety and efficacy of low-power slow-heating diode laser-induced photocoagulation of prostatic tissue for treatment of benign prostatic hyperplasia, we conducted a series of acute and chronic studies using a diode laser (810 nm) to irradiate human prostate. Methods: The study included 6 patients undergoing radical prostatectomy because of malignancy. Laser radiation to the prostate was given transurethrally, in a noncontact mode, at the time of the procedure in 2 patients (acute group): 10 W for 300 seconds on the right side of the prostate and 15 W for 180 seconds on the left. In the remaining 4 patients (chronic group), laser radiation was given at 1, 7, 10, and 12 weeks prior to the prostatectomy. Results: In the acute group the average depth of coagulation was 8.5 and 9.0 mm for the laser regimens of 15 W for 180 seconds and the 10 W for 300 seconds, respectively. In the chronic group, the average depth of coagulation was 8.9 mm for both laser regimens studied. In the acute group, there was an ill-defined hemorrhagic ring at the periphery of the lesion. At 1 week, an intact necrotic coagulum was present. At 7 weeks, some of the coagulated tissue had already sloughed off. Longer follow-up at 10 and 12 weeks demonstrated formation of a well-defined cavity with mostly re-epithelialized surface. Conclusions: Our findings suggest that noncontact diode laser (810 nm) irradiation can induce in the human prostate significant coagulation necrosis followed by sloughing of tissue and cavitation of the prostatic urethra. © 1995.