Highly crystalline Ca-Al layered double hydroxides (LDH) covered with needle-like hydroxyapatite (HA) nanocrystals was prepared from Ca-Al-Cl LDH by an alkaline treatment at 40 degrees C using a Na(3)PO(4) solution at pH 9.0. The HA nanocrystals had lengths of 100-200 nm, widths of 30 nm, and were grown on the surface of hexagonal LDH plate crystals. A 80-nm-thick nanolayer was observed between the HA and LDH, and was found to contain calcium and phosphate, but no aluminum or chloride. This layer was initially formed by the anion exchange reaction of phosphate ions in solution with chloride ions in the LDH crystals. The LDH structure was retained during this reaction. The elution of calcium ions from the LDH basal layers initiated the precipitation of the HA crystals on the surface. The stability of LDH in hydrochloric acid (pH 3) was dramatically improved by the presence of the nanolayers. The LDH-HA nanocomposites are acid-resistant and have potential in areas such as the sustainable removal of harmful anions and cations and as drug delivery carriers.