ObjectiveHigh fluoride concentration treatments are known to react with enamel and dentine forming calcium fluoride (CaF2)-like deposits, but strategies to improve this reactivity beyond increasing fluoride concentration/reducing pH in fluoride treatments have not been explored. Here we investigated the ability of a calcium pre-treatment to improve fluoride reactivity. DesignIn a blind and randomized in vitro study, sound and carious enamel and dentine slabs (n = 11/group) were randomly allocated into one of the following treatments: Deionized water (negative control); 0.05% sodium fluoride (F, positive control); 150 mM calcium lactate solution followed by 0.05% sodium fluoride solution (Ca→F); 150 mM calcium lactate solution premixed with 0.05% sodium fluoride solution (CaF2, active control). Alkali-soluble fluoride (representing CaF2-like deposits formed on the substrates) was extracted from the slabs using 1 M KOH for 24 h and measured by an ion-specific electrode. Carious slabs were further observed under scanning electron microscopy (SEM). Data were analyzed by two-way ANOVA followed by Tukey test. ResultsThe Ca→F treatment enhanced fluoride reactivity with all tested substrates when compared with F alone. Carious substrates had a greater reactivity with F and Ca→F than their respective sound substrates, confirming that increased porosity enhances the reactivity with fluoride. Alkali-soluble fluoride concentration after the CaF2 treatment did not differ among the different substrates, suggesting this treatment causes only contamination with preformed CaF2, which was noted under SEM. ConclusionA calcium pretreatment enhances the reactivity of fluoride with enamel and dentine.