Canonical Wnt signaling is central to normal bone homeostasis and secretion of Wnt signaling inhibitors by myeloma (MM) cells contributes to MM-related bone resorption and disease progression.The aim of this study was to test the effect of Wnt3a on bone disease and growth of MM cells in vitro and in vivo.Although Wnt3a activated canonical signaling in the majority of MM cell lines and primary cells tested, Wnt3a had no effect on MM cell growth in vitro.Moreover, forced expression of Wnt3a in H929 MM cells conferred no growth advantage over empty vector transfected cells in vitro or importantly when grown subcutaneously in SCID mice.Whereas H929 cells stably expressing an empty vector injected into human bone grew rapidly and induced a marked reduction in bone mineral density, bones engrafted with Wnt3a-expressing H929 cells were preserved, exhibited increased osteoblast/osteoclast ratios, and reduced tumor burden.