OBJECTIVE The utility of subperiosteal tissue expansion (STE) for mandibular augmentation with hydroxylapatite (HA) particles was evaluated experimentally in two groups of dogs. METHODS Eight grams of solid HA particles were implanted in previously formed bilateral subperiosteal expanded beds on the mandible. In one group, the HA particles were implanted in the intact subperiosteal expanded bed and in the other the newly formed capsule overlying the bone was removed before implantation of HA. RESULTS Despite marked augmentation with a large number of HA particles, there was neither deformity nor infection of the implants, which maintained the original desired contour. The firm fibrous capsule formed by STE allowed consolidation of the HA particles and prevented their migration. In the first group, although histologic evidence of bony union of the implanted particles was not seen even after 2 months, clinical immobilization of the grafts was achieved in 1 to 2 months. After 1 year, substantial bone growth into the implants was observed. In the second group, clinical immobilization was observed even after 2 weeks. Histologic evidence of bony union of the particles located along the surface of the mandible was observed after 1 or 2 months. In these specimens, however, marked bone resorption of the mandibular cortex, was also noted. No evidence of bone or osteoid formation on the expanded periosteal side was noted in either group. CONCLUSIONS These results provide fundamental data regarding mandibular augmentation using STE, and indicate that the subperiosteal expanded bed is an excellent recipient site for onlayed implants.