Postmenopausal osteoporosis is characterized by increased bone resorption due to estrogen deficiency. Receptor activator of nuclear factor-kappaB-Fc (RANK-Fc), a fusion protein that specifically blocks receptor activator of nuclear factor ligand binding to RANK, has been known to be efficient and well tolerated in animal models of osteoporosis. Here we show that cell-based gene therapy with RANK-Fc effectively prevented bone loss in ovariectomized (OVX) mice. Thirty-one young adult female C57Bl/6 mice were used, and repeated intraperitoneal injection of mesenchymal stem cells (MSCs) transduced with retrovirus was performed as follows: 1) Sham-operated mice (n = 8); 2) OVX mice treated with phosphate-buffered saline (OVX-PBS; n = 8); 3) OVX mice injected with MSCs transduced with control retrovirus (OVX-green fluorescent protein [GFP]; n = 7); and 4) OVX mice injected with MSCs transduced with RANK-Fc (OVX-RANK-Fc; n = 8). Cellular expression of RANK-Fc was confirmed by Western blot analysis of cell lysates and conditioned medium and also by enzyme-linked immunosorbent assay for the mice serum. Measurement of bone mineral density (BMD) by dual-energy x-ray absorptiometry (PIXImus) revealed that the OVX-RANK-Fc group gained significantly higher BMD than either the OVX-PBS group or OVX-GFP group after 8 weeks. The expression of GFP, which is coexpressed with RANK-Fc, was detected by polymerase chain reaction analysis of DNA isolated from femur and intra-abdominal fat, whereas no GFP signal was identified in liver, brain, heart, lung, or bone marrow aspirates. These suggest that expression of RANK-Fc by genetically modified MSCs may be a feasible option for the prevention of bone loss induced by ovariectomy.