The purpose of this study was to compare different parameters of Er:YAG laser irradiations on the marginal microleakage of Class V resin composite restorations. A total of 45 extracted premolars were selected for the study. Class V cavities prepared on both buccal and lingual surfaces of teeth by Er:YAG laser or bur and divided into nine groups. The occlusal margins were in enamel and the cervical margins were in cementum. Group-1: bur preparation; Group-2: laser preparation (lp) (600mJ/5Hz); Group-3: lp (300mJ/10Hz); Group-4: lp (200mJ/15Hz); Group-5: lp (150mJ/20Hz); Group-6: lp (200mJ/20Hz); Group-7: lp (300mJ/14Hz); Group-8: lp (400mJ/10Hz); Group-9: lp (700mJ/5Hz). All teeth were stored in distilled water at 37 degrees C for 24h, then thermocycled 1,000 times (5-55 degrees C). Five teeth from each group were chosen for the microleakage investigation and two teeth for the scanning electron microscope (SEM) evaluation. Teeth, which were prepared for the microleakage test were immersed in 0.5% methylene blue dye for 24h. After immersing, the teeth were sectioned and observed under a stereomicroscope for dye penetration. Data were analyzed by Kruskal-Wallis and Mann-Whitney U tests (p<0.05). More microleakage was observed in cervical regions compared to occlusal regions in all groups (p<0.05) except for Group 6 (p>0.05). No significant difference was observed among groups in terms of occlusal and cervical surfaces, separately (p>0.05). It may be concluded that the cavities prepared by Er:YAG laser showed higher degree of microleakage than bur prepared at cervical regions. Different parameters of Er:YAG laser irradiations affected microleakage. SCANNING 38:434-441, 2016. (c) 2016 Wiley Periodicals, Inc.