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Treatment of the immature pulpless tooth presents both an endodontic and restorative challenge. A more favorable long-term prognosis may be achieved with a mineral trioxide aggregate (MTA) apexification procedure followed by an internal bonding technique. We investigated the efficacy of this treatment option by testing the sealing ability and retention characteristics of MTA when placed as an apical barrier in a standardized in vitro open apex model. MTA was placed as an apical barrier at a thickness of 1 mm or 4 mm, with and without prior calcium hydroxide medication. The barriers were challenged with bacteria exposure within a leakage model and displacement forces on an Instron machine. In the leakage study, 100% of the MTA apical barriers showed bacterial penetration by day 70, compared with 20% of MTA root-end fillings used as controls. The displacement study demonstrated a statistically significant greater resistance to force with a 4-mm thickness of MTA, regardless of calcium hydroxide use. We concluded that it was the intracanal delivery technique and not the MTA that contributed to the leakage observed. MTA shows promise in our proposed treatment option of immature pulpless teeth if the sealing ability can be enhanced by improving the delivery technique.