Nanocrystalline plate-shaped particles of hydroxyapatite, with ca. 300 nm edges were directly precipitated from dilute calcium chloride and sodium phosphate solutions, at ambient temperature and pH 7.4. This pH was selected to make the hydroxyapatite appropriate for medical applications. The precipitated crystallites were loosely aggregated in spherulites of 2-4mum dimensions. The direct precipitation of hydroxyapatite was achieved by submitting the aqueous solutions of calcium and phosphate to microwave irradiation immediately after mixing. The nanocrystals were remarkably stable, not requiring any liquid or stabilizing agent, remaining free flowing after 3 years of storage in non-hermetically closed containers. An explanation for the direct precipitation of hydroxyapatite from water solution, which allows the formation of nano-particles is suggested. (C) 2002 Elsevier Science B.V. All rights reserved.