Although recent data shows that arsenic trioxide (As2O3) is capable of inducing cell death via cell cycle arrest and apoptosis both in acute promyelocytic leukemia (APL) and in non-APL cells, the mechanisms of As2O3-mediated cell death are not fully understood. In this study, we investigated the in vitro effects of As2O3 on cell growth inhibition and cell death in human T-lymphocytic leukemia and myelodysplastic syndrome (MDS) cell lines. As2O3 significantly inhibited the proliferation of Molt-4 and Mutz-1 cells in dose- and time-dependent manner. Autophagic cell death (programmed cell death type II) and apoptosis (programmed cell death type I) were activated together in leukemia cell lines after exposed to As2O3. Numerous large cytoplasmic inclusions and vacuoles were observed in As2O3-treated cells using electron microscope. Furthermore, 3-methyladenine (an autophagy inhibitor) significantly reduced autophagic cell death and sequentially induced apoptosis. Finally, leukemia cells treated with 4 microM As2O3 showed a considerable up-regulation of Beclin-1 (a Bcl-2-interacting protein) expression, which was independent of transcription of mRNA and required protein synthesis. In addition, Molt-4 cells treated with As2O3 exhibited the down-regulation of Bax protein expression, suggesting that Bax may be involved in accumulating of Beclin-1 and triggering autophagic cell death in As2O3-treated leukemia cells. These results may lead to a better understanding of the mechanism of action of As2O3, and provide a suggestion that As2O3 may be of therapeutic value for the treatment of patients with human T-lymphocytic leukemia and myelodysplastic syndrome.