Microleakage occurs due to the lack of sealing ability of root sealing materials or adhesive cements used to lute post-core materials. This may cause premature failure of endodontic treatment. The purpose of this study was to evaluate regional bond strengths of four adhesive systems to root canal dentin. Sixteen extracted human single rooted teeth were used. After removing the crowns and the pulp tissues, the root canals were then instrumented and irrigated with or without 5% NaOCl. The root canal dentin walls were then bonded with C&B Metabond, Panavia F, Variolink II, or Rely-X. Microtensile bond strengths to root canal dentin were then measured using an Instron machine. The bond strength data were recorded and expressed in MPa. Statistically significant differences were found among the NaOCl treated and nontreated groups (p < 0.05). C&B Metabond gave the highest bond strength compared with others in the control group (p < 0.05). C&B Metabond also had significantly higher bond strength compared with Variolink II and Panavia F groups when the canals were irrigated with NaOCl (p < 0.05). Copyright © 2003 by The American Association of Endodontists.