Calcium hydroxide has been used as a liner in resin composite restorations to protect the pulp. Recent research has demonstrated that pulpal inflammation is caused by microleakage of restorations and by the subsequent passage of bacteria. The present study involved scanning electron microscopic observation of cross-sections of resin composite-dentin interfaces after the interposition of a layer of calcium hydroxide. A new-generation adhesive system that involves etching of the dentin was used. Ultrastructural analysis indicated that polymerization shrinkage of the resin composite caused the separation of the calcium hydroxide from the dentinal surface, forming 8- to 15-um-wide interfacial gaps in 100% of the areas studied. Where the adhesive was applied directly to dentin, it adhered closely, forming a gap-free attachment with evidence of an acid-resistant hybrid layer (4 to 6 um in thickness) and resin tags of various lengths that hermetically sealed the dentinal tubules.