AbstractThe crystallographic features of various histological sites found in enamel carious lesions were studied by means of X‐ray microbeam diffraction. The most interesting finding was that the crystallinity of apatite crystals in the intact surface layer covering the demineralized lesions was higher than that of the crystals in the subsurface lesions or unaffected areas. Acid‐dissolution experiments using synthetic hydroxyapatite pellets showed that a well mineralized layer was produced on the pellet surface under the condition that no Ca and PO4ions were added to the initial acidic solution. Furthermore, X‐ray diffraction analysis revealed that half‐line breadth values of 004, 222, and 310 reflections of apatite pellets decreased, and their integrated peak intensities increased with dissolution time. These findings are in agreement with the results obtained in the study of enamel carious lesions, leading to the conclusion that the intact surface layer is formed by deposition of the mineral ions from dissolving subsurface lesions, and that this process is accompanied by the improvement in crystallinity of apatite crystals, possibly due to growth of the c