A glass ionomer cement is formed from the reaction of calcium aluminosilicate glass powder containing fluoride and a polyacrylic acid. A clear advantage offered by glass ionomer cements is one similar to that of the silicate cements, namely of releasing, by diffusion, fluoride ions. We see thus a substantially increase of the amount of fluoride and, consequently, the resistance to caries of the enamel adjacent to restorations with such cements. The amount of fluoride released is highest during the first 24 hours, beginning with the mixing, and it dramatically decreases over the next 2 or 3 days. Within 1 or 2 weeks, the amount released stabilizes, showing little change, but continues to decrease slowly. Several studies have demonstrated that fluoride release from glass ionomer cement, is effective in inhibiting secondary caries in vitro.