Porous ceramics consisting of hydroxyapatite (HA) are useful scaffolds for hone regeneration. Their pore structures can he designed by the size and morphology of the HA particles which construct the porous ceramics. We previously reported that the hydrothermal treatment of alpha tricalcium phosphate (a-TCP) porous ceramics produced porous ceramics consisting of rod-shaped HA crystals. The purpose of this study is to investigate the effects of kinds of solutions for the hydrothermal treatment of the a-TCP porous ceramics. The a-TCP porous ceramics were hydrothermally treated with various aqueous solutions containing NaCl, CaCl2, Na3P0 4, Na2HP04, NaH2P04, or H3P04 at 150 C for 10 h. The a-TCP phase was transformed into HA by the hydrothermal treatment in ultrapure water. NaCI, CaCl 2, Na3PO4 or Na2HPOA solutions. to form porous ceramics consisting of rod-shaped HA particles. The size of the rod-shaped HA particles formed in NaiPOJ or Na2HP04 solutions was smaller than that formed in ultrapure water, NaCI. or CaCl2 solutions. When the NaCl or CaCl2 solutions were used, chloride ions were incorporated into HA. The α-TCP phase of the porous ceramics was transformed into a hiphase consisting of HA and monetite (DCPA) after treatment in NaH2P04 solution, and into DCPA in the H3P04 solution. resulting in porous ceramics consisting of plate-shaped particles. The solutions used for the hydrothermal treatment governed the resultant pore structures of the HA porous ceramics.