Recently, strategies for acute myeloid leukemia (AML) therapy have been developed that target anti-apoptotic BCL2 family members using BH3-mimetic drugs such as ABT-737. Though effective against BCL2 and BCL-X L, ABT-737 poorly inhibits MCL-1. Here we report that, unexpectedly, ABT-737 induces activation of the extracellular receptor activated kinase and induction of MCL-1 in AML cells. MEK inhibitors such as PD0325901 and CI-1040 have been used successfully to suppress MCL-1. We report that PD0325901 blocked ABT-737-induced MCL-1 expression, and when combined with ABT-737 resulted in potent synergistic killing of AML-derived cell lines, primary AML blast and CD34+ 38-123+ progenitor/stem cells. Finally, we tested the combination of ABT-737 and CI-1040 in a murine xenograft model using MOLM-13 human leukemia cells. Whereas control mice and CI-1040-treated mice exhibited progressive leukemia growth, ABT-737, and to a significantly greater extent, ABT-737+ CI-1040 exerted major anti-leukemia activity. Collectively, results demonstrated unexpected anti-apoptotic interaction between the BCL2 family-targeted BH3-mimetic ABT-737 and mitogen-activated protein kinase signaling in AML cells: the BH3 mimetic is not only restrained in its activity by MCL-1, but also induces its expression. However, concomitant inhibition by BH3 mimetics and MEK inhibitors could abrogate this effect and may be developed into a novel and effective therapeutic strategy for patients with AML.