Clinical Relevance Use of the centripetal open-sandwich technique may allow for placement of a Class II resin composite restoration with better marginal adaptation, fewer voids and reduced microleakage than the closed sandwich technique. SUMMARY Purpose: This study evaluated whether a Class II restoration in a flowable resin composite has to be placed prior to (open-sandwich technique) or after (closed-sandwich technique) construction of the interproximal wall in the centripetal build-up technique in order to reduce microleakage. Methods and Materials: Thirty non-carious molars were selected and randomly divided into two groups (n=15). A standardized Class II preparation was made with the cervical margin 1 mm below the cementum-enamel junction. In Group 1, flowable resin composite was applied as a 1 mm base, remaining exposed at the cervical margin. In Group 2, the hybrid resin composite was applied to the interproximal wall, followed by a layer of flowable composite on the pulpal floor, away from the margins. The restorations were then subjected to 500 thermal cycles, each with a dwell time of 20 seconds at 5°C and 55°C. Adaptation at the cervical margin was evaluated by dye penetration and SEM analysis using the replica technique. The data were statistically analyzed using the Mann-Whitney U-test ( p <0.05). Results: The centripetal open-sandwich technique led to significantly lower dye penetration than the centripetal closed-sandwich technique ( p <0.001). Conclusion: Flowable resin composite placed under hybrid resin composites in Group 1 provided better marginal adaptation and fewer voids. However, neither Group 1 nor Group 2 was able to completely prevent microleakage.