Purpose: The aim of this study was to compare marginal microleakage of resin composites in Class I and Class V cavities prepared classically (bur) or with an Er,Cr:YSGG laser system. Materials and Methods: Fifty-four caries-free extracted human third molars were divided into two equal groups: group 1 (unlased), all cavities (n = 9 Class V; n = 18 Class I) were prepared with a bur and total-etched; group 2 (lased), all caivities (n = 9 Class V; n = 18 Class I) were prepared with an Er,Cr:YSGG laser. After cavity preparation, an adhesive system (Excite) was applied and a resin composite (Tetric Ceram) placed into all cavities according to manufacturer’s instructions. The teeth were sealed with nail polish, immersed in a 5% methylene blue, sectioned, and examined with a stereomicroscope at 10X magnification. Dye penetration scores were recorded using different scales for Class I and Class V cavities. Results: Unlased Class I cavities had statistically significantly (p < 0.001) less microleakage than lased Class I cavities. In Class V cavities, conventional preparation also yielded significantly less microleakage than did Er,Cr:YSGG laser preparation. Conclusion: Our findings show that Er,Cr:YSGG laser irradiation of enamel and dentin is not a valid alternative to acid-etching pretreatment of cavities to be filled with resin composite materials.