Gutta-percha (G) and sealer do not resist coronal leakage. In this study, the effect of orifice plugs using dentin-bonding/composite resin(C) or IRM on coronal leakage was evaluated in vivo. Sixty-one premolar roots in three beagle dogs were instrumented and filled with G and AH 26(S), or G alone. The coronal 2 mm was replaced with C or IRM, or left untreated. The access cavities were kept open for 8 months, the dogs were killed and the periapical regions of the roots were histologically examined. Periapical inflammation was observed in 89% of the group without plugs, but in those with plugs, the occurrence was decreased to 39% (C + G + S), 38% (IRM + G + S) and 58% (C + G), respectively. Furthermore, severe inflammation was observed in 50% of the former group but only 0 to 17% of the latter. The substantial reduction in apical periodontitis by the use of coronal plug underscores the clinical importance of providing an additional barrier to coronal leakage in comparison to that provided by gutta-percha and sealer alone.