Macrophage inflammatory protein–1α (MIP-1α) gene expression is abnormally regulated in multiple myeloma (MM) owing to imbalanced expression of the acute myeloid leukemia–1A (AML-1A) and AML-1B transcription factors. We hypothesized that the increased expression ratios of AML-1A to AML-1B also induced abnormal expression of other hematopoietic and bone-specific genes that contribute to the poor prognosis of MM patients with high levels of MIP-1α. We found that interleukin-3 (IL-3) was also induced by the imbalanced AML-1A and AML-1B expression in myeloma. IL-3 mRNA levels were increased in CD138⁺ purified myeloma cells with increased AML-1A–to–AML-1B expression from MM patients, and IL-3 protein levels were significantly increased in freshly isolated bone marrow plasma from MM patients (66.4 ± 12 versus 22.1 ± 8.2 pg/mL; P = .038). IL-3 in combination with MIP-1α or receptor activator of nuclear factor–kappa B ligand (RANKL) significantly enhanced human osteoclast (OCL) formation and bone resorption compared with MIP-1α or RANKL alone. IL-3 stimulated the growth of interleukin-6 (IL-6)–dependent and IL-6–independent myeloma cells in the absence of IL-6, even though IL-3 did not induce IL-6 expression by myeloma cells. These data suggest that increased IL-3 levels in the bone marrow microenvironment of MM patients with imbalanced AML-1A and AML-1B expression can increase bone destruction and tumor cell growth.