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The present study evaluates the morphological changes in root canal walls and temperature changes at root surfaces as a result of intracanal irradiation by erbium, chromium:YSGG laser under various conditions in vitro. Sixty single-rooted human teeth were examined. Root canals were prepared, and laser irradiation was performed using an optic fiber at output powers ranging from 1 to 6 W with or without water spray cooling. Specimens were evaluated by stereoscopy, scanning electron microscopy, and thermography. Carbonization and cracks were observed in all samples irradiated without cooling, whereas little or no carbonization and no smear layer or debris were observed in samples irradiated with cooling. Maximum temperature rise at irradiation without cooling was above 37°C, whereas that at irradiation with cooling was 8°C. Results of the present study indicate that erbium, chromium:YSGG laser irradiation with water spray cooling is a useful method for removal of smear layer and debris from root canals.