The formation of hydroxyapatite (HAp) from amorphous calcium phosphate (ACP) was in situ observed in calcium-rich aqueous solutions (Ca/P molar ratios were 1.67, 1.83, and 2.0) by freeze-drying the precipitates withdrawn at selected time intervals during the reaction. As the more excess calcium ions exist in the solution, the HAp crystallization from ACP occurred more rapidly and the Ca/P molar ratio of the finally obtained precipitates increased to reach the stoichiometric value of 1.67. Acicular HAp nanocrystals grew from the inter-particle phase between the spherical particles within the ACP aggregates in an initial stage of the phase transformation. This observation is in favour of the view that the internal rearrangement process is responsible for the ACP-HAp transformation - it is against the opinion suggesting a dissolution-reprecipitation process. (c) 2004 Elsevier B.V. All rights reserved.