Mesenchymal stem cells (MSCs) are multipotent progenitor cells that can differentiate into several cell types. Bone marrow (BM)-MSCs mainly differentiate into osteoblasts or adipocytes. MSC interactions with their microenvironment directly affect their self-renewal/differentiation program. Here, we show for the first time that Fas ligand (FasL), a well-explored proapoptotic cytokine, can promote proliferation of BM-derived MSCs in vitro and inhibits their differentiation into adipocytes. BM-MSCs treated with a low FasL dose (0.5?ng/ml) proliferated more rapidly than untreated cells without undergoing spontaneous differentiation or apoptosis, whereas higher doses (25?ng/ml) induced significant though not massive BM-MSC death, with surviving cells maintaining a stem cell phenotype. At the molecular level, 0.5?ng/ml FasL induced ERK1/2 phosphorylation and survivin upregulation, whereas 25?ng/ml FasL induced caspase activation. Importantly, 25?ng/ml FasL reversibly prevented BM-MSC differentiation into adipocytes by modulating peroxisome proliferator-activated receptor gamma (PPAR?) and FABP4/aP2 expression induced by adipogenic medium. All such effects were inhibited by anti-Fas neutralizing antibody. The in vitro data regarding adipogenesis were confirmed using Fas(lpr) mutant mice, where higher PPAR? and FABP4/aP2 mRNA and protein levels were documented in whole tibia. These data show for the first time that the FasL/Fas system can have a role in BM-MSC biology via regulation of both proliferation and adipogenesis, and may have clinical relevance because circulating Fas/FasL levels decline with age and several age-related conditions, including osteoporosis, are characterized by adipocyte accumulation in BM.