BACKGROUND To develop a new procedure for laser prostatectomy through a biopsy needle, we evaluated the efficacy of interstitial laser irradiation with a pulsed Ho:YAG laser (lambda: 2.1 microns) in the normal canine prostate. METHODS Pathological studies were performed up to 6 months after interstitial laser irradiation in 26 mongrel dogs (15.4 +/- 2.6 kg). Two dogs underwent the identical procedures without irradiation as controls (control group). Prostate glands of the other 24 dogs were irradiated interstitially with a Ho:YAG laser through a silica glass fiber (400 microns phi). In one prostate, each lobe was irradiated with 500 J/cm2 or 150 J/cm2 in energy density (fluence). Four prostates were irradiated with 150-175 J/cm2 in fluence (low-fluence group). Nineteen prostates were irradiated with 500-600 J/cm2 in fluence (high-fluence group). Prostates glands were excised immediately, 1 month, 2 months, 3 months, or 6 months after irradiation and examined histologically. To investigate the influence of interstitial irradiation with a Ho:YAG laser on smooth muscle and/or on the adrenergic activity of the prostate, we also measured changes in intraluminal pressure of the prostatic urethra upon administration of epinephrine (10 micrograms/kg) in six dogs before and after irradiation. We developed a novel three-membrane balloon catheter with an open end for measuring this pressure change. RESULTS Specimens harvested immediately in control and low-fluence groups showed slight hemorrhage in areas surrounding the punctured or irradiated site, but the surrounding smooth muscles or small vessels were not destroyed. In the high-fluence group, smooth muscles and small vessels approximately 1 mm thick that surrounded the ablated hole were destroyed, but the thickness of the thermal coagulation layer was only approximately 0.2 mm. These findings suggested that damage to the small vessels and smooth muscle may have been caused by laser-induced stress waves rather than by a thermal effect. In the control and low-fluence groups, prostatic volume remained unchanged 1 month after puncture or low-fluence irradiation. In the high-fluence group, huge cavities were created in the laser-irradiated prostate gland 1 or 2 months after irradiation and prostatic volume was reduced maximally by 60%. In the high-fluence group, the cavities collapsed and prostatic volume was reduced by up to 50% at 6 months post-procedure. The prostatic urethra expanded following the reduction in volume. The maximal intraluminal change in pressure of the prostatic urethra decreased significantly after laser irradiation (p < 0.05). CONCLUSIONS Results indicate that interstitial irradiation of the prostate with a Ho:YAG laser with high-fluence may relieve the anatomical and functional obstruction of the prostatic urethra in benign prostatic hyperplasia.