PURPOSE: Nuclear factor-kappaB (NF-kappaB), activated in multiple myeloma (MM) cells by microenvironmental cues, confers resistance to apoptosis. The sesquiterpene lactone parthenolide targets NF-kappaB. However, its therapeutic potential in MM is not known. Experimental Designs: We explored the effects of parthenolide on MM cells in the context of the bone marrow microenvironment. RESULTS: Parthenolide inhibited growth of MM cells lines, including drug-resistant cell lines, and primary cells in a dose-dependent manner. Parthenolide overcame the proliferative effects of cytokines interleukin-6 and insulin-like growth factor I, whereas the adhesion of MM cells to bone marrow stromal cells partially protected MM cells against parthenolide effect. In addition, parthenolide blocked interleukin-6 secretion from bone marrow stromal cells triggered by the adhesion of MM cells. Parthenolide cytotoxicity is both caspase-dependent and caspase-independent. Parthenolide rapidly induced caspase activation and cleavage of PARP, MCL-1, X-linked inhibitor of apoptosis protein, and BID. Parthenolide rapidly down-regulated cellular FADD-like IL-1beta-converting enzyme inhibitory protein, and direct targeting of cellular FADD-like IL-1beta-converting enzyme inhibitory protein using small interfering RNA oligonucleotides inhibited MM cell growth and lowered the parthenolide concentration required for growth inhibition. An additive effect and synergy were observed when parthenolide was combined with dexamethasone and TNF-related apoptosis-inducing ligand, respectively. CONCLUSION: Collectively, parthenolide has multifaceted antitumor effects toward both MM cells and the bone marrow microenvironment. Our data support the clinical development of parthenolide in MM therapy.