Eleven patients who previously had undergone elective facial osteotomy and in whom blocks of porous hydroxyapatite were implanted into osteotomy gaps later consented to open biopsy of the implant material. A total of 24 biopsies were harvested at a mean time of 10.2 months following implantation. Gross anatomic findings were recorded at the time of biopsy. Specimens were harvested from the zygomatic buttress of the maxilla (12), the piriform buttress of the maxilla (4), the maxillary interdental premolar region (2), and the anterior mandible (6). Histologic sections were examined undecalcified using a modified trichrome stain. Eight of the 11 patients were followed radiographically for a minimum of 24 months. In the absence of infection, there was rapid fibrovascular ingrowth and, provided there was contact with host bone, bone ingrowth. This was observed in 21 of 24 biopsy specimens. The extent of bone ingrowth, as judged qualitatively, was highly variable and did not correlate with the time interval from implantation, anatomic site, or surface area of bone-implant interface. A healing process involving an osteoid phase of bone maturation and suggestive of continued net bone production was consistently found. The gross anatomic, radiographic, and histologic findings are discussed and provide further insight into the biologic behavior of porous block hydroxyapatite implanted into the maxillofacial skeleton.