Introduction: The most common method used in the clinical setting to bond orthodontic brackets is pre-etching with 37% orthophosphoric acid and bonding using various adhesives. In recent years laser was proposed for pre-treatment of the enamel surfaces for orthodontic bracket bonding. Laser irradiation causes thermally induced changes on the enamel surface, such as roughening and microirregularities, similar to those caused by acid etching. Aim: The aim of this study was to compare the shear bond strength values of orthodontic brackets luted with RMGIC after enamel etching by Er:YAG laser operated using a digitally controlled handpiece (X-Runner) in two different working modes, versus a conventional etching protocol including phosphoric acid, after accelerated artificial aging/thermal cycling of specimens. Materials and methods: Sixty healthy human premolars were randomly allocated to three experimental groups (n = 20) and etched with: Group 1: Er:YAG laser in the Super Short Pulse (SSP) mode (100 mJ, 20 Hz, 2 W); Group 2: Er:YAG laser in the Quantum Square Pulse (QSP) mode (120 mJ, 10 Hz, 1.2 W), using a digitally controlled handpiece (X-Runner); Group 3 (control group): 5.25% sodium hypochlorite pre-treatment and, then 37% phosphoric acid for 15 s. Stainless steel brackets were bonded using light-curing RMGIC for orthodontic bonding. After the term cycling (1800 cycles), the shear bond strength (SBS) testing was performed by a universal testing machine. After debonding, both enamel and bracket surfaces were examined with a stereomicroscope at magnifications of 25x (the whole bonded surface was observable in the microfield) and 50x (detail of the debonded surface) to determine the amount of the luting material still present on the surfaces. Evaluation and scoring of the luting material remnants were carried out by the same observer, blinded regarding the surface treatment or the outcome of the study, under 25x magnification. The specimens were randomly evaluated and a score from 0% to 100% in 5% increment was given for the amount of the remaining luting material on the examined surfaces. Results: Group 3 surfaces gave the lowest mean SBS (10.6104 +/- 2.66196 MPa), while Group 1 provided the highest one (13.1795 +/- 3.37904 MPa), which was significantly different from the control group (Group 3, p = 0.0226). Group 2 provided intermediate values (11.8486 +/- 0.59832 MPa), which were not significantly different from those of the control group or from SSP (p = 0.4215 and p = 0.3082, respectively). When analysing the amount of the remaining luting material on the examined surfaces, a significant negative linear correlation was found between the percentage of material still attached to enamel and the one attached to the brackets, when R square value of the linear fit was 0.7834, showing a significant (ANOVA, p < 0.001) and strong correlation. When analyzing each experimental group separately, a very strong significant correlation could be found for the control (Figure 2) and the SSP surface treatments (Figure 3) (R square = 0.9864 and 0.9774, respectively) with linear fits (control: % enamel = 99.9355 - 0.9987*% bracket; SSP: % enamel = 105.9 - 0.9964*% bracket) very close to an expected theoretical equation of: % enamel = 100 - % bracket. QSP group (Figure 4), however, provided a significant (p = 0.0002), but relatively weak correlation (R square = 0.554) with a linear fit equation: % enamel = 100.0267 – 0.7604747*% bracket. Conclusion: Laser treatment in SSP mode of enamel surfaces provided a similar shear behaviour of the luting material to the conventional acid-etching procedures, and shear bond strength was increased by approximately 24%. QSP-mode laser treatment did not significantly alter shear bond strength values when compared to laser in SSP mode or to acid-etch. It nevertheless provided a different shear behaviour, where the percentage of the luting material remaining on the bracket and on the enamel did not correlate well, due to many chippings and cohesive fractures inside the luting material. Both laser treatments provided an increased percentage of remaining cement on enamel surfaces after bracket detachment, suggesting an enhancement of bond strength at enamel/cement interface when compared to acid-etching. Within the limitations of an in vitro study, laser etching of the enamel for orthodontic bonding represents a viable alternative to acid etching.