Mesenchymal stromal cells (MSCs) show significant immune-suppressive properties both in vitro and in vivo. Based on their immune-stealth properties, allogeneic MSCs are used to treat several diseases, for example the injection of MSCs in infarcted heart tissue or their use in bone-cartilage regeneration. The most spectacular treatment was recently described. MSCs were able to down-regulate the severity of graft-versus-host disease, leading to an impressive 20 to 50% increase in the two-year survival of bone marrow transplantation patients. Here the current literature is reviewed to elucidate the different mechanisms involved in these two clinical treatment modalities of MSCs.