Calcium hydroxyapatite (CaHAP) was prepared by homogeneous precipitation from aqueous solutions in the presence of formamide (FA) and Cl- ions. The structures and compositions of the particles formed under various conditions were examined by various methods. The crystal phase and morphology of the particles depended on the concentrations of the reagents, especially FA, in the starting solutions. Spherical, Ca-deficient CaHAP particles with diameters less than 800 mu m were obtained with 3-7 mol/dm(3) FA and consisted of needle-like particles growing in a radial manner. Cl- ions contributed to the crystal growth of these CaHAP particles.