Arsenic trioxide (As(2)O(3)) has been reported to have potent antitumor effects in vitro and in vivo by inducing cell death via cell cycle arrest and apoptosis in leukemia cells, but the mechanisms of As(2)O(3)-mediated cell death are not fully understood. In this study, we provided in vitro evidence that As(2)O(3) was a potent inducer of autophagy in leukemia K562 and its drug-resistant line K562/ADM cells. As(2)O(3) significantly activated autophagic cell death (programmed cell death type II) in leukemia cell lines. Numerous large cytoplasmic inclusions, abundant autophagic vacuoles, phagocytizing cytoplasm and organelles were observed in As(2)O(3)-treated cells using electron microscope. MDC-labeled autophagic vacuoles were observed by fluorescent inverted phase contrast microscopy and the enhanced MDC fluorescent staining was detected by flow cytometry in As(2)O(3)-treated cells. Furthermore, real-time quantitative RT-PCR revealed that the expression levels of Beclin-1 and LC3 genes, which play key roles in autophagy, increased in As(2)O(3) treated samples than in controls, indicating that autophagy can potentially be involved in the antitumor properties of As(2)O(3). The expression level of Bcl-2 gene, an anti-apoptotic molecule, decreased in As(2)O(3) treated samples than in controls, suggesting that Bcl-2 may be involved in accumulating Beclin-1 and triggering autophagic cell death in As(2)O(3)-treated leukemia cells. Western blotting also showed that As(2)O(3) up-regulated Beclin-1. Altogether, our data provide direct evidence that autophagic cell death is critical for the effects of As(2)O(3) on acute myelogenous leukemia cells.