The embryonic origin of lymphatic endothelial cells (LECs) has been a matter of controversy since more than a century. However, recent studies in mice have supported the concept that embryonic lymphangiogenesis is a complex process consisting of growth of lymphatics from specific venous segments as well as the integration of lymphangioblasts into the lymphatic networks. Similarly, the mechanisms of adult lymphangiogenesis are poorly understood and have rarely been studied. We have recently shown that endothelial progenitor cells isolated from the lung of adult mice have the capacity to form both blood vessels and lymphatics when grafted with Matrigel plugs into the skin of syngeneic mice. Here, we followed up on these experiments and studied the behavior of host leukocytes during lymphangiogenesis in the Matrigel plugs. We observed a striking co-localization of CD45⁺ leukocytes with the developing lymphatics. Numerous CD45⁺ cells expressed the LEC marker podoplanin and were obviously integrated into the lining of lymphatic capillaries. This indicates that, similar to inflammation-induced lymphangiogenesis in man, circulating CD45⁺ cells of adult mice are capable of initiating lymphangiogenesis and of adopting a lymphvasculogenic cellular differentiation program. The data are discussed in the context of embryonic and inflammation-induced lymphangiogenesis.