Purpose: The purpose of this study was to evaluate the ability of newer fluoride-releasing restorative materials to protect the root surface from acid attack. Materials and Methods: The materials used were glass-ionomer cement (GIC), resin-modified glass-ionomer cement (RM-GIC), and a compomer (Comp). A composite resin (CR) was used as the control. The restored teeth were stored in deionized, distilled water for 14 days and subjected to 300 thermocycles (55°C and 5°C). The teeth were cycled in a demineralizing solution (pH 5.0 or 4.0) for 6 hours and in a remineralizing solution (pH 7.0) for 17 hours for 10 days. The depths of lesions created by acid challenge were measured at the interface of the tooth and the restorative material and then at a distance of 50, 100, and 300 µm from the tooth-restoration margin using polarized light microscopy and contact microradiography. Results: At pH 4.0, there was significant difference in the depth at the interface between the tooth and the restorative material (P .001). The GIC and RM-GIC were protective, and the lesion depths were significantly shallower than for Comp or CR. The protective effect varied depending on the distance from the interface of the tooth and the restorative material. At pH 5.0, the GIC and RM-GIC had no lesions at the interface, while the Comp and the CR had lesions (P .001). Conclusion: Fluoride-releasing glassionomer cement seems to be an appropriate material to seal the root canals of overdenture abutments, because it has an inhibiting effect on demineralization at the cavity wall in vitro.