Triptolide, a diterpenoid isolated from the Chinese herb Tripterygium wilfordii Hook.f, has shown anti-tumor activities in a broad range of solid tumors. Here we examined its effects on leukemic cells and found that, at 100=nM, it potently induced apoptosis in various leukemic cell lines and primary AML blasts. We then attempted to identify its mechanisms of action. Triptolide induced caspase-dependent cell death accompanied by a significant decrease in XIAP levels. Forced XIAP overexpression attenuated triptolide-induced cell death. Triptolide also decreased Mcl-1, but not Bcl-2 and Bcl-XL levels. Bcl-2 overexpression suppressed triptolide-induced apoptosis. Further, triptolide induced loss of the mitochondrial membrane potential and cytochrome C release. Caspase-9-knockout cells were resistant, while caspase-8-deficient cells were sensitive to triptolide suggesting criticality of the mitochondrial, but not the death receptor pathway for triptolide-induced apoptosis. Triptolide also enhanced cell death induced by other anti-cancer agents. Collectively, our results demonstrate that triptolide decreases XIAP and potently induces caspase-dependent apoptosis in leukemic cells mediated through the mitochondrial pathway at low nanomolar concentrations. The potent anti-leukemic activity of triptolide in vitro warrants further investigation of this compound for the treatment of leukemias and other malignancies.