High levels of interleukin-7 (IL-7) have been associated with bone loss due to its stimulatory osteoclastogenic activity. Osteolytic patients' peripheral blood mononuclear cells (PBMCs) differentiate into osteoclasts without adding stimulating factors. Now, we investigated the potential role of IL-7 in the spontaneous osteoclastogenesis occurring in these patients. We identified significant differences in serum IL-7 levels between patients with/without bone metastases, suggesting that IL-7 might be effective as a clinical marker of disease progression. In patients' PBMC cultures we demonstrated that IL-7 stimulates osteoclastogenesis by inducing TNF-alpha release by T and B cells. These findings add further details to the disclosure of the mechanisms controlling bone metastases in solid tumors.