Multiple myeloma (MM)-induced osteoclast (OC) formation occurs in close contact with MM cell infiltration into the bone marrow (BM) due to the imbalance of the receptor activator of NF-kappa-B ligand (RANKL)/osteoprotegerin (OPG) ratio in favor of RANKL in the micorenvironment. Soluble factors including CCL3/MIP-1?, IL7 and IL-3 also contribute to the increased OC formation in MM.The immunomodulatory drugs (IMiDs) directly inhibit OCs, however their effect on the mechanisms involved in MM-induced OC formation are not known and have been investigated in this study. We found that both Lenalidomide (LEN) and Pomalidomide (POM), at concentration ranging reached in vivo, significantly blunted RANKL up-regulation normalizing the RANKL/OPG ratio in human BM osteoprogenitor cells (PreOBs) co-cultured with MM cells and inhibited CCL3/MIP-1? production by MM cells. The reduction of CD49d expression on MM cells, a molecule critically involved in RANKL up-regulation in the micorenvironment, accompanied this effect. Consistently the pro-osteoclastogenic property of the conditioned medium of MM cells co-cultured with PreOBs was reduced in the presence of both IMiDs. By microarray analysis we further investigated the effect of POM and LEN on the transcriptional profile of both MM cells and PreOBs. We found a significant down-regulation in MM cells, in addition to CD49d, of genes belonging to the adhesion molecules family such as ITGA8 and ICAM2 (CD102) induced by both IMiDs compounds. In conclusion our data suggest that POM and LEN inhibits MM-induced OC formation through the inhibition of RANKL/OPG ratio targeting the expression of adhesion molecules by MM cells.