Various cytokines and cytokine antagonists hold promise as new therapeutic agents for osteoporosis, but their application is hindered by delivery problems. Gene transfer offers an attractive technology with which to obviate these restrictions. Its utility was evaluated in an animal model of osteoporosis. Disease was induced by surgical ovariectomy and monitored by measuring bone weight after 12 days, and by histomorphometry after 5 weeks. Genes were transferred to the mice by intramedullary injection of adenoviral vectors. LacZ and luciferase marker genes were used to identify the bone marrow cells transduced by this procedure, and to track the possible spread of transgenes to other organs. The effect on bone loss of transferring a cDNA encoding the human interleukin-1 receptor antagonist (IL-1Ra) was then evaluated. The intramedullary injection of adenoviral vectors transduced lining osteoblasts, osteocytes and cells within the bone marrow. Luciferase activity persisted within the injected femora and adjacent musculature for at least 3 weeks, and in the draining lymph nodes for 2 weeks. Transient, low level expression was present in the liver, but no luciferase was detected at any time in the lung or spleen. Intramedullary introduction of the IL-1Ra gene resulted in circulation of the corresponding protein at concentrations that peaked on day 3, and returned to baseline by day 12. Transfer of the IL-1Ra gene strongly reduced the early loss of bone mass occurring in response to ovariectomy. Furthermore, it completely inhibited the loss of matrix detected by histomorphometry at 5 weeks. The protective effect of this gene was not restricted to bones receiving intramedullary injection of the vector, but occurred in all bones that were evaluated. This proof of concept encourages further development of gene therapy approaches to the treatment of osteoporosis.