The aim of this study was to evaluate the microleakage of all-in-one self-etch adhesives and their respective nanocomposites in class V cavities prepared by erbium:yttrium-aluminum-garnet (Er:YAG) laser and bur. Class V cavities were prepared on both buccal and lingual surfaces of 72 premolars by Er:YAG laser or bur and divided into six groups (n = 24). The occlusal margins were enamel and the cervical margins were cementum. The groups were as follows: group 1 Er:YAG laser preparation (E) + Xeno V (X) + CeramX (C); group 2 bur preparation (B) + X + C; group 3 E + AdheSE One (A) + Tetric EvoCeram (T); group 4 B + A + T; group 5 E + Clearfil S3 Bond (CSB) + Clearfil Majesty Esthetic (CME); group 6 B + CSB + CME. All teeth were stored in distilled water at 37A degrees C for 24 h, then thermocycled 500 times (5-55A degrees C). Ten teeth from each group were chosen for the microleakage investigation and two teeth for the scanning electron microscopy (SEM) evaluation. The teeth that were prepared for the microleakage test were immersed in 0.5% basic fuchsin dye for 24 h. After immersion, 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). Bur-prepared cavities presented less microleakage in all groups for enamel (P < 0.05); however, in cervical margins, there were no differences between laser-prepared and bur-prepared cavities in the Xeno V + CeramX and AdheSE One + Tetric EvoCeram groups (P > 0.05). SEM observations of restorative material-dentin interfaces seemed to correspond with those of the microleakage test. Microleakage at the cervical interfaces was greater than that at the occlusal interfaces. Er:YAG laser-prepared class V cavities yielded more microleakage in occlusal margins with all-in-one self-etch adhesives and the respective manufacturer's nanocomposites.