Two different types of vascularized rib grafts presently are used in clinical practice and as experimental models for investigations on free microvascular bone transfer: the posterior rib graft, including both medullary and periosteal blood supply to the bone; and the posterolateral segmental rib graft, supplied by periosteal vessels alone, Complete survival of bone after successful revascularization of the posterior type of graft is well established, but this graft has the disadvantage of a complicated dorsal dissection which has limited its clinical use. Instead, many microsurgeons have utilized the posterolateral rib segment, which is easy and safe to excise although its viability and adequate microcirculation have not yet been confirmed. In nine large dogs, we compared the viability and vascularity of bone after transfer of the two types of bone grafts by histological methods, fluorochrome bone-labeling, microangiography, and technetium scintigraphy. The grafts were transferred to the subcutaneous fat tissue in the groin, where blood supply was reconstituted by microvascular anastomoses to local donor vessels. The results suggest that a bone transplant with revascularization of periosteal only established a collateral circulation to medullary vessels, and that there is no difference in viability of the two kinds of grafts. Clinical Relevance: The technique of transferring whole bone segments by microvascular anastomoses of their vascular pedicles has been employed clinically either by preserving the periosteal blood supply alone or by preserving the medullary and the periosteal blood supply. This study demonstrates that the preservation of the periosteal blood supply alone can result in complete bone-graft survival even when the graft is placed in a poorly vascularized tissue bed.