Bioactive glass nanoparticles synthesized by flame spray synthesis were tested for their remineralization capabilities in vitro. After artificial demineralization with EDTA, human dentin was treated with 20-50nm size bioactive glass nanoparticles or a micrometer-sized, commercial reference material (PerioGlas) for up to 30 days. The degree of remineralization was measured using quantitative gravimetric methods (thermogravimetry, elemental analysis) and element-sensitive scanning electron microscopy imaging to detect new mineral precipitated on or within the demineralized tooth matrix. After treatment with bioactive glass nanoparticles for 10 or 30 days a pronounced increase in mineral content of the dentin samples suggested a rapid remineralization. The mechanical properties of the remineralized dentin samples were well below the stability of natural dentin. It is suggested that this lack of mechanical reconstitution may be attributed to an imperfect arrangement of the newly deposited mineral within the demineralized tooth matrix. Nevertheless, the substantially higher remineralization rate induced by nanometer-sized vs. micrometric bioactive glass particles corroborated the importance of particle size in clinical bioglass applications.