Secondary caries is one of the most important factors leading to replacement of dental restorations. This investigation assessed the capacity of fluoride-releasing restorative materials to resist caries in vitro. Class 5 cavities were prepared in buccal and lingual surfaces of 18 extracted premolars. The six materials used were: high-copper amalgam (Dispersalloy), fluoride-containing amalgam (Fluor-Alloy), composite resin (Valux), fluoride-containing composite resin (Heliomolar), glass-ionomer cement (Fuji), and glass-ionomer cement with silver particles added (Ketac-Silver). After 15 weeks in an acid gel for caries-like lesion formation, the teeth were sectioned longitudinally and examined with polarized light. The results showed that restoration of caries with glass-ionomer materials and fluoride-containing amalgam may be of great importance in the prevention of both secondary caries around the restorations and primary caries in surface enamel adjacent to the restoration.