The purpose of the present study was to investigate whether fluoride (F) ions are really capable of the repair (remineralization) of damaged crystals and useful for reinforcing the quality (i.e. modify the crystal structure) of tooth enamel using transmission electron microscopy and Raman microprobe analysis. Additionally, carbonic anhydrase activity was measured in immature enamel tissue to compare the harmfulness of F ions to that of cadmium (Cd) ions during the process of crystal nucleation by means of differential gas pressure analysis. Electron micrographs indicated no signs of remineralization of artificially damaged crystals after incubation in a remineralizing solution and further revealed that treatment with acidulated phosphate fluoride (APF) gel caused crystal dissolution rather than crystal improvement. Regarding crystal structure modification, Raman microprobe analysis revealed that no up-shift of PO₄^3-ν₁ band assigned to human sound enamel crystals occurred when APF gel was used. Furthermore, fluorapatite crystals were not generated by daily intake of F ions in developing rat tooth enamel. A differential gas pressure method demonstrated that the harmfulness of F exposure during the nucleation process of calcified hard tissues was much greater than that of Cd exposure. These results demonstrate that F treatments have no effect on improving crystal quality or remineralization and are inconsistent with the purpose of public health.