We recently demonstrated that the local transplantation of human peripheral blood (PB) CD34+ cells, an endothelial/hematopoietic progenitor cell rich population, contributes to fracture repair via vasculogenesis/angiogenesis and osteogenesis. Human PB mononuclear cells (MNCs) are also considered as a potential cell fraction for neovascularization. We have previously shown the feasibility of human PB MNCs to enhance fracture healing. However, there is no report directly comparing the efficacy for fracture repair between CD34+ cells and MNCs. In addition, an unhealing fracture model, which does not accurately resemble aclinical setting, was used in our previous studies. To overcome these issues, we compared the capacity of human granulocyte colonystimulating factor-mobilized PB (GM-PB) CD34+ cells and human GM-PB MNCs in a nonunion model, which more closely resembles a clinical setting. First, the effect of local transplantation of 1x105 GM-PB CD34+ cells (CD34+ group), 1x107 GM-PB MNCs (containing approximately 1x105 GM-PB CD34+ cells) (MNC group) andphosphate-buffered saline (PBS) (Gibco, Grand Island, NY) (PBS group) on nonunion healing was compared. Similar augmentation of blood flow recovery at peri-nonunion sites wasobserved in the CD34+ and MNC groups. Meanwhile, a superior effect on nonunion repair was revealed by radiological, histological and functional assessment in the CD34+ group compared with the other groups. Moreover, through in vivo and in vitro experiments, excessive inflammation induced by GM-PB MNCs was confirmed and believed to be one of the mechanisms underlying this potency difference. These results strongly suggest that local transplantation of GM-PB CD34+ cells is a practical and effective strategy for treatment of nonunion after fracture.