We studied the effect of the irrigant temperature (5°-38°C) and flow rate (100-500 mL/min) on the extent of coagulation necrosis induced during Nd:YAG laser irradiation in the canine prostate model. The tissue response was quantified based on histopathologic evaluation of the lesions. Changing the irrigant temperature or the flow rate within the ranges studied did not significantly affect the size of the thermal lesions. The intraprostatic temperature distribution during laser irradiation is mainly governed by blood perfusion rather than by irrigant variables. © 1995, Mary Ann Liebert, Inc. All rights reserved.