The author studied the validity of bone scintigraphy in the assessment of viability and anastomotic patency of free bone grafts revascularized by microvascular anastomoses and also compared them with conventional free devascularized bone grafts and osteotomies. Sixty rabbits were divided into three different groups, and scintigraphy was carried out using technetium labeled methylene diphosphonate on revascularized living bone grafts, conventional devascularized bone grafts and osteotomies. Of these, 38 rabbits could be followed up to 6 weeks. Analyzing the histologic, radiographic and scintigraphic data, the following results were obtained. 1. Free living bone grafts, revascularized by periosteal blood supply from the posterior tibial artery, revealed incomplete bone cell survival on histologic sections taken at 6 weeks after operation. In the osteotomy group the distal fragments were completely viable. Living bone grafts with failed anastomosis and dead bone grafts were almost completely necrotic. 2. Radionuclide uptake in dead bone grafts and living bone grafts with failed anastomosis increased gradually up to 6 weeks after operation. Radionuclide uptake in living bone grafts with patent anastomosis increased rapidly during the first 2 weeks, and increased gradually thereafter up to 6 weeks. In the osteotomy group radionuclide uptake also increased rapidly during the first 2 weeks but decreased slowly thereafter up to 6 weeks. 3. Radionuclide uptake at 6 weeks after operation became comparable to that of the adjacent host bone and there was no significant difference among the various groups. On the basis of the experimental results, the author derived following conclusions. 1. The differences of radionuclide uptake among the various living and dead bone grafts were most remarkable at 2 weeks following the grafting procedure. The radioactivity was highest following osteotomy which preserved blood supply, and it was lower in the living bone grafts with patent anastomosis, while that in the dead bone grafts and living bone grafts with failed anastomosis was lowest. 2. The time factor in performing scintigraphy was most important in assessing the viability of bone grafts and anastomotic patency. The optimal time to assess the viability and anastomotic patency appeared to be 2 weeks after grafting.