Two patients were treated with surgical implantation of synthetic, granular, nonporous hydroxylapatite to correct bone defects around dental implants. Ultrastructural examination of biopsy specimens taken several months later showed that the hydroxylapatite material was encapsulated mostly by fibrous connective tissue, without any signs of active inflammation. Osteoid and mature bone were also present in many areas. Under the electron microscope, an electron-dense delimiting band was detected at the interface of the tissue and the material. A polygonal, networklike structure, originating from the electron-dense layer and penetrating into the periphery of the hydroxylapatite granules, was also observed. These findings confirm the results of previous studies, in which it was hypothesized that the electron-dense structures were of a mucopolysaccharide nature and might prove the close bond between the hydroxylapatite and the tissues.