SummaryThe permeability, temperature and morphologic changes of the wall of the root canal induced by Nd:YAG, CO2and argon lasers were studied. The changes were evaluated according to the presence or absence of a smear layer. Root canals of 140 human single‐rooted teeth were enlarged using a step‐back technique. Permeability was evaluated by the extent of methylene blue dye penetration into the tubules. Temperature changes were measured using a thermovision system, and morphological changes were evaluated by scanning electron microscopy. Laser energy was delivered into the canal by means of a flexible optical fibre or metal tip. There were statistically significant differences in permeability between Iased groups with and without a smear layer in the cervical third of the root canal following lasing. In the middle third of the root canal, all three laser types induced permeability increases in groups with a smear layer. In the apical third, statistically significantly decreases in permeability were observed among C02 laser and Nd:YAG compared with control group (P<0.01). Rises in temperature ranged from a minimum of +10.1 °C (CO2laser) to a maximum of +54.8 °C (argon laser). All three laser devices appeared capable of producing a glazed‐like surface and