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Decreased activity of osteoblasts (OB) contributes to osteolytic lesions in multiple myeloma (MM). Dickkopf-1 (DKK1) is a soluble Wnt inhibitor produced by MM cells that inhibits osteoblastogenesis in the bone marrow (BM) microenvironment. DKK1, also present in MM patient plasma, has been shown to inhibit the differentiation of osteoblast precursor cells in vitro, and its plasma level correlates with focal bone lesions in MM. Therefore, we have evaluated DKK1 as a target in MM in the context of the BM microenvironment. We first analyzed the effects of a DKK1 neutralizing antibody (BHQ880) on OB differentiation. Anti-DKK1 Ab was able to increase differentiation of mesenchymal stem cells (MSCs) to OB and reduce IL-6 levels following MSC differentiation to OB. While OB activity in MSCs cultured with osteoblast differentiation media was reduced in the presence of INA-6 MM cells, treatment with DKK1 neutralizing antibody was able to restore OB activity in a dose-dependent manner, overcoming the negative effect of MM cells. We did not observe a direct effect of this Ab on growth or survival of human MM cell lines. However, when a panel of MM cell lines was cultured with BM stromal cells (BMSC), the Ab induced significant growth inhibitory effects on MM cells, associated with downregulation of IL-6 produced by MSCs. To evaluate the in vivo bone and antitumor effects of anti-DKK1 Ab treatment on MM cells in the human microenvironment, we used a SCID-hu murine model, using INA-6 MM cells. In this model we observed a direct correlation between the level of soluble human DKK1 in murine blood and the tumor growth. Following treatment with BHQ880, by bone histology analysis, we observed increased trabecular bone and numbers of OBs in the retrieved bone chip from BHQ880 treated mice compared to control mice, one month after initial dose. We also found an increase in human osteocalcin in the serum of BHQ880-treated mice compared to the controls, suggesting an increase in OB activity. Interestingly, we observed suppression of myeloma growth, measured by changes in serum level of soluble huIL6sR, 4 weeks following BHQ880 treatment of the mice. These results support DKK1 as an important therapeutic target in myeloma and provide the rationale for clinical evaluation of this molecule in MM to improve bone disease and to inhibit MM growth.