Mesenchymal stem cells (also known as multipotent stromal cells, MSCs) are considered as promising candidate cells for stem cell-based therapy. However, the applications of MSCs are facing controversial concerns of potential tumorigenic risks. There is also increasing evidence that MSCs may play a modulatory role in the development and progression of tumors. MSCs have the potential to migrate to tumor sites and promote tumor cell proliferation, invasion and metastasis. In addition to these risks, MSCs also have shown to be an attractive target for gene/cell-mediated anti-tumor therapy. These complicated behaviors of MSCs in cancer warrant further study to evaluate the benefits of MSCs treatment and the long-term risk of tumor origin or incidence from MSCs under different pathological conditions.