Bone marrow-derived, CD34+ progenitor cells have been shown to promote the repair of damaged tissues, offering promise for the treatment of hereditary and acquired human diseases. These cells in fact differentiate into endothelia, hematopoietic cells and possibly neurons, fibroblasts and muscle. CD34+ and AC133+ progenitor cells may participate in neovascularization by differentiating into endothelial cells. Circulating bone marrow-derived endothelial cells home to sites of neovascularization and stimulate healing of injured tissues but also promote restenosis, tumor growth and inflammatory disease. These cells may thus participate in tissue regeneration or pathogenesis of several diseases. Although the molecular mechanisms that promote the homing and recruitment of bone marrow-derived progenitor cells to remodeling tissues remain unclear the evidence that these cells promote tissue repair is strong.