Changes in the bone mineral content (BMC) of edentulous mandibles after augmentation with porous hydroxyapatite (HA), in blocks (B) in one side and in granules (G) in the opposite side, were measured in vivo by dual-photon absorptiometry. The BMC changes in each side at the 1-, 2-, and 3-year follow-up were expressed in percent of the 1-week postoperative BMC value. In unexposed augmentations new bone formation seems to occur during the first year in the HA-B sides, followed by the biologic age-related BMC loss; while the HA-G sides show pronounced resorption of hydroxyapatite and bone during the entire period. Although porous HA-B and HA-G are unsuitable for residual ridge augmentation, porous HA-B may be used for implantation in areas without risk for exposure of HA resulting from loading of covering soft tissues. Schlagwörter: bone changes, bone mineral content, dual-photon absorptiometry, hydroxyapatite, implants, mandible, residual ridge augmentation, surgery