Research has provided evidence that tumor growth depends on the interaction of tumor cells with stromal cells, as already suggested in 1889 by Paget. Experimental and clinical studies have revealed that tumor stromal cells can be derived from bone marrow (BM)-derived progenitor cells, such as mesenchymal stern cells (MSCs), which can be mobilized into the circulation and incorporate into tumor micro-environments. Many observations indicate that, in the tumor micro-environment, MSCs have several tumor growth promoting functions, including expression of growth factors, promotion of tumor vessel formation and creation of tumor stem cell niches. This review will discuss the currently known tumor growth promoting BM-derived cells and focus on the role of MSCs in modulating tumor micro-environments. In addition, we will discuss the potential of inhibiting BM-derived cells and their utilization as cellular vehicles for selective delivery of cancer therapeutics as additional strategies in the treatment of cancer. (C) 2008 Elsevier Ireland Ltd. All rights reserved.