This study aimed to examine the suitability of the diode laser with a wavelength of 810 nm for reducing bacteria in root canals. Forty-four extracted, endodontically prepared teeth were inoculated with Escherichia coli and Streptococcus faecalis. No bacterial growth was observed when the teeth were irradiated at an output power of 4 W and a pulse rate of 10 milliseconds, with breaks of 10 millise conds between the irradiations. Exposure time was five times 5 seconds. Infrared spectroscopic examinations revealed that irradiation at 4 W under even circling movements resulted in a maximum rise in temperature at 6 degrees C on the root surface. Stain penetration tests and scanning electron microscopic examinations revealed complete closure of the dentinal tubules on the irradiated root canal walls.