Induction of angiogenesis is critical for the functional restoration of tissue perfusion upon ischemia. Growing evidence supports the concept that leukocytes and especially monocytes promote hypoxia-mediated angiogenesis upon tissue ischemia. Here, we identified midkine (MDK), a novel proangiogenic factor in human neutrophils and monocytes by means of western blotting technique as well as by confocal microscopy. This finding was confirmed by immunoelectron microscopy. Upon hypoxia (1% O2) for 4 h, MDK was found to be up-regulated in neutrophils and monocytes. When compared to normoxic conditions, substantial amounts of MDK were secreted by monocytes stimulated by hypoxia for 4 h as measured by ELISA technique. The biological relevance of these findings was studied in vivo by measuring sprouting angiogenesis in the chicken chorioallantoic membrane assay. Here, MDK was found to induce angiogenesis in a dose dependent manner. The response induced by MDK was similar in size compared to the effect observed upon stimulation by vascular endothelial growth factor. Taken together, we identified the novel and potent proangiogenic factor MDK in human leukocytes which is up-regulated upon hypoxia. This mechanism may be involved in hypoxia-induced angiogenesis upon tissue ischemia. Supported by DFG (WA 1048/1–1 and 1–2).