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Purpose: To compare marginal leakage and gap formation in large resin composite Class II cavities with their gingival margins in cementum, using three different additional retentions in the proximal box. Methods: Standardized large Class II MOD cavities with gingival margins in cementum were prepared in 40 recently extracted molars and divided into four groups according to their retention in the proximal box: (G1) no retention; (G2) vertical grooves in the buccal and lingual walls; (G3) "pot holes" in the gingival wall and (G4) horizontal grooves in the gingival wall. All groups were restored with the incremental technique using the same resin composite (QuiXfil, Dentsply) and a bonding agent (Prime Bond NT, Dentsply). After polishing, all of the restored teeth were immersed in dye solution and submitted to simultaneous cyclic loading. Impressions of the gingival margins were made before and after loading, and epoxy resin replicas were evaluated for gap formation using a scanning electron microscope. The microleakage and gap extension data were evaluated by ANOVA and Tukey's test (p<0.05). Gap extension before and after mechanical loading was compared by Student's t-test. A correlation analysis was made between the gap extension and microleakage (Pearson's correlation test). Results: All groups with additional retention (G2=0.565, G3ffi0.346 and G4=0.078) showed fewer gap formations than the control group (G1=2.076). Similar results were found for microleakage tests. All groups presented an increase in gap extension after loading, with the exception of the group with gingival retention grooves (G4). G4 showed the best results in relation to both parameters. There was a low correlation between gap extension and microleakage. [ABSTRACT FROM AUTHOR]