Radiographic and histometric evaluation of a new form of bonegraft substitute derived from reef-building sea coral was performed in a canine diaphyseal defect model. Comparably sized blocks of this material and iliac crest autograft were placed into bilateral surgically created cortical windows in the distal radial diaphyses of 14 dogs. Representative graft specimens of both types underwent densitometric radiography and harvesting at 3, 6, 12, 24, and 48 months. Histometric analysis of implant specimens revealed satisfactory union and native osseous ingrowth at all time intervals. Graft specimens exhibited adequate union with scant host bone ingrowth at three months, but subsequent progressive appositional closure of cancellous spaces during the ensuing 45 months. Volume fraction and mean width of host bone remained stable in the implants but increased in the grafts due to neocortex formation. Stereologic distribution of bone was homogeneous in both implant and graft specimens, and no evidence for biodegradation of the former was observed. Corrected transmission density determinations accurately reflected these differences but correlated significantly with volume fractions and tissue widths of bone and soft tissue only in the autografts. These results support the successful early application of coralline hydroxyapatite bone graft substitutes as an alternative to autogenous grafting in the clinical setting and emphasize the potential role of noninvasive densitometric techniques in monitoring the incorporation of bone-graft materials.