Characterization of synthetic enamel formed by newly developed dental paste for repairing early caries lesions was performed using scanning and transmission electron microscopies and atomic force microscopy, with the chemical composition analysis using X-ray photoelectron spectroscopy. It was shown that the hydroxyapatite nanocrystals seamlessly grew in the interface between the paste and tooth enamel, and arranged their (0 0 0 1) face parallel to the tooth surface same as enamel apatite. The rapid growth of high crystalline large hydroxyapatite crystals by applying the paste is attributed to the dissociation of calcium phosphate clusters, Ca-9(PO4)(6), which is the main growth unit of hydroxyapatite when it grows under physiological conditions, to ionic species in the concentrated and low pH phosphate solution. (c) 2005 Elsevier B.V. All rights reserved.