We have investigated the osteoconduction in porous hydroxyapatite ceramics grafted into the defect of the lamina of dogs in expansive open-door laminoplasty. Porous Hydroxyapatite block (HAB) was sintered at 900 degrees C with size of 4 x 7 x 19 mm, a mean pore diameter of 90 microns and a void volume of 70%. The operation was performed in 29 dogs. The animals were sacrificed at three or at six months after the operation (the three-month group and the six-month group). After the grafted HABs with adjacent vertebrae were removed, the stability of the grafted HABs was manually evaluated. The HAB was firmly fixed to the laminae in 14 of 20 cases. It seemed that the stability of the HAB was poor in those cases in which the HAB had not been tightly sutured. To investigate osteoconduction, serial sections of the laminar bones including the HAB were prepared without decalcification and were examined histologically. New bone formation in the pores of the HAB was found in all cases. However, the osteoconduction rate was higher in the six-month group than in the three-month group (p < 0.01). And the osteoconduction rate was higher in those cases in which the HAB was firmly fixed to the laminae than in those with a loosely fixed HAB. Bone ingrowth in the HAB was frequently observed in the zone adjacent to the bone, more so than in other zones. Osteoconduction appeared to occur at the contact surface and to extend towards the center of the HAB.