The growth and metastasis of solid tumors not only depends on their ability to escape from immune surveillance but also hinges on their ability to invade the vasculature system as well as to induce the formation of new blood vessels. Gr-1(+)CD11b(+) myeloid-derived suppressor cells (MDSCs), overproduced in tumor-bearing hosts, contribute significantly to all these aspects. They also have a potential role in the osteolysis associated with bone metastases. They are formidable partners in tumor metastasis.