The objective of the study was to corrtpare the extent of thermocoagulation induced in the human prostate by two rgiiaens of Nd:YAG laser (5OWx6Os and l5Wxl8Os) and one regimen of diode (BlOnm) laser (l5Wxl8Os) . Powers were measured at the fiber tip. The study was conducted in 6 patients undergoing radical prostatectomy because of malignancy whowe prostates were treated transurethrally with laser radiation at the time of the procedure. The prostate specimens were retrieved approximately 90 minutes after laser treatment, and then prepared for histological examination. In all the cases, the diode laser was applied to the left side of the prostate and the Nd:YAG laser (either regimen) to the right side, in order to prevent treatment variables related to histological composition. Coagulation necrosis was confined within the transition zone in all the case and at times BPH was unaffected, if nodular. Similar depth of coagulation was observed for both Nd:YAG regimens (mean 12mm, range 10 to 14) and the depth of coagulation was slightly less for the diode laser (mean 10.6mm, range 8 to 14). Our results suggest that, at the studied dosimetries, the thermocoagulation effect of diode laser in the human prostate approximates to that seen with Nd:YAG laser.