Jeffrey A. Seaman, DVM; G. Elizabeth Pluhar, DVM, PhD; Nathan D. Rose, DVM; Jeff I. Weinstein, DVM; Hongfei Guo, PhD; Kari L. Anderson, DVM American Journal of Veterinary Research September 2009 70 9 1079-1086 Objective—To evaluate scintigraphy, radiography, and histomorphometric analysis for assessing incorporation of intercalary bone grafts and to compare incorporation of cortical autografts and allografts by the recipient. Animals—12 skeletally mature sheep. Procedures—A 5-cm tibial defect was filled with a cortical allograft (n = 6) or autograft (6) and stabilized with an interlocking nail. Radiography, scintigraphy, and fluorochrome bone labeling were performed every 3 months for 24 months. Radiographic evaluation included grading of the host and graft union and assessment of implants and grafts. Technetium-99m-hydroxymethylene diphosphonate radionuclide uptake was measured. Sheep were euthanatized 24 months after surgery, and bone formation was evaluated via histomorphometric analysis of fluorochrome labeling. Results—Complete union was detected on radiographs by 21 months in all sheep but developed earlier in sheep that received an autograft versus in those that received an allograft. Radionuclide uptake peaked at 3 months and returned to presurgical values at 12 months. Histomorphometric analysis revealed fluorochrome labeling corresponding to each time point, with most bone formation at 9 through 15 months. Scintigraphy findings did not correlate well with fluorochrome labeling of newly formed bone. Conclusions and Clinical Relevance—Although bone production around cortical bone grafts was detected by use of scintigraphy, this method did not provide accurate assessment of graft incorporation in sheep. Furthermore, bone produced by activated periosteum could not be distinguished scintgraphically from bone that replaced the graft. Intercalary autografts healed more rapidly and had greater incorporation into the host bone, compared with findings for allografts.