The resistance to arsenic trioxide (ATO) treatment is relatively common (55 to 80%) in MM patients.In this report, we show that ATO at clinically achievable concentrations (2-7 μM) activated p38 MAPK in both myeloma cell lines and primary myeloma cells, a finding not previously well-documented in myeloma cells.Inhibition of p38 MAPK activation by pharmacologic inhibitors (SB203580) or down-regulation of p38 MAPK by siRNA significantly increased the apoptosis and/or growth inhibition induced by ATO treatment in myeloma cells.Combination of ATO and p38 MAPK inhibition abolished the IL-6 enhanced protection of myeloma cells against ATO treatment.The ATO resistant cell line developed in our laboratory showed an increase in p38 MAPK activation.The increase of apoptosis by the combination of ATO and SB203580 was accompanied by the activation of caspase-9 and easpase8 suggesting that both extrinsic and intrinsic apoptotic pathways are involved.Additionally, the p38 MAPK activation by ATO was associated with increased phosphorylation and up-regulated expression of Hsp27.These results suggest that ATO-induced p38 MAPK activation plays an important role in the resistance to ATO in myeloma cells and that p38 MAPK inhibition may overcome resistance to ATO treatment in myeloma patients.