OBJECTIVE To evaluate in vitro the effect of cavity preparation with microburs and diamond-coated ultrasonic tips on the microleakage and marginal fit of six end-root filling materials. STUDY DESIGN. The following materials were assessed: amalgam (Amalcap), zinc oxide eugenol (IRM), glass ionomer (Vitrebond), compomer (Cavalite), mineral particle aggregate (MTA) and composite (Clearfil). Cavity preparation was performed with microburs or diamond ultrasonic tips in single-root teeth. The seal was evaluated in two experiments: a microleakage assay on the passage of dye to the interface; and a scanning electron microscopy study and analysis of epoxy resin replicas, measuring the size of gaps in the interface between filling material and cavity walls. Multifactorial ANOVA, multiple comparison test and Student 's t test were used for statistical analyses of the data, considering p <0.05 to be significant. RESULTS Clearfil and MTA achieved a hermetic seal. Leakage and interface gap size was greater with Cavalite than with Clearfil and MTA, followed by Vitrebond and IRM. The worst seal was obtained with Amalcap. The use of diamond-coated ultrasonic tips improved the seal and reduced the gap when using materials that did not hermetically seal the cavity (Amalcap, IRM, Cavalite, Vitrebond). The preparation technique did not affect materials that achieved a hermetic seal (Clearfil, MTA). CONCLUSIONS Clearfil and MTA obtained a hermetic seal due to their excellent marginal fit and are the most recommended materials for clinical use, taking account of their sealing capacity. Ultrasonic cavity preparation is preferable because it improves the seal and marginal fit of materials that do not achieve a hermetic seal of the cavity (Amalcap, IRM, Cavalite, Vitrebond).