Osteoporosis is a skeletal disorder characterized by reduced bone mineral density (BMD) and increased risk of fracture. We studied the effects of cell therapy of human adipose tissue-derived stromal cell (ADSC) on ovariectomy-induced bone loss in T cell deficient nude mice. Twelve-week-old female nude mice underwent ovariectomy and were treated with ADSC, estrogen, or phosphate buffered saline (PBS). Whole body BMD revealed that treatment of ADSC was more protective against ovariectomy-induced attenuation in bone mass gain compared with PBS control after cell therapy (8.4±1.1 vs. 2.4%±1.4%, p<0.05 at 4 weeks, 13.7±1.3 vs. 7.7%±1.8%, p<0.05 at 8 weeks) and this effect was comparable to that of estrogen. μCT analysis revealed that the effect of ADSCs was specific to trabecular bone. Serum osteocalcin levels were increased 4 weeks after ovariectomy and treatment with ADSCs (76.4±11.6 ng/mL) increased osteocalcin to a greater extent when compared with estrogen (63.1±6.7 ng/mL, p<0.05) or PBS treatment (58.0±9.2 ng/mL, p<0.05). Flow cytometry analysis for PKH26-labeled ADSCs and quantitative real-time PCR analysis for human β-globin from bone revealed that transplanted ADSCs were trafficking in bone 48 h after injection and subsequently disappeared. There was no evidence of long-term engraftment of infused ADSCs in bone. In vitro, treatment with ADSC-conditioned medium enhanced osteogenic differentiation in stromal cells and preosteoblasts. These results suggest that cell therapy of ADSCs protects against ovariectomy-induced bone loss in nude mice in a paracrine manner.