Purpose: Vertical ridge augmentation is a technique to enhance alveolar bone growth or to correct bone defects. However, its long-term predictability and stability are still unknown. The aim of this study was to evaluate the impact of three alloplastic grafting materials on the retention of vertically augmented bone tissue. Materials and Methods: Four titanium caps (3 mm in radius), filled with different alloplastic materials (resorbable hydroxyapatite [HA], porous or nonporous nonresorbable HA) or peripheral blood (control) were placed on the calvaria of seven male New Zealand rabbits. Three months after implant placement, mucoperiosteal flaps were raised to expose the defect sites. The titanium caps were carefully removed and the tissue was covered by repositioned flaps. Two months later, the animals were sacrificed and the augmented new bone was retrieved and prepared for histomorphometric examination. The highest point of each augmented area was measured directly with a Boley gauge. Bone healing was evaluated by a semiquantitative bone score according to the relative proportion of newly formed bone in the cap (0%, 25%, 50%, and 75% of new bone). Results: All substitutes promoted supracranial bone augmentation. Some specimens from nonresorbable HA-augmented tissues fractured at the time of cap removal, indicating that these newly augmented tissues were fragile and less flexible. Height of the augmented tissue for nonresorbable HA, resorbable HA, and controls was 2.3, 1.5, and 1.4 mm, respectively. Most augmented tissue contained less than 25% new bone. Conclusions: Data from this experiment indicated that, while all materials conducted new bone formation predictably, new tissue augmented by resorbable hydroxyapatite and control sites were less stable. Further investigations are encouraged to search for new materials and techniques, which enhance the long-term stability of vertically augmented tissue. Keywords: bone formation, vertical ridge augmentation, alloplasts, long-term stability