Chronic lymphocytic leukemia (CLL) cells are characterized by defective apoptosis which leads to their extended survival. Arsenic trioxide (As(2)O(3)) was reported to induce cell death in many malignant cells, but the specific pathway of As(2)O(3)-induced apoptosis/necrosis remains controversial. Our aim was to determine if As(2)O(3) kills CLL cells through apoptosis and whether this is accompanied by reduction in Bcl-2 levels. Cells from nine patients with CLL were incubated with increasing concentrations of As(2)O(3) (0.5-2 microM) for 2, 7, or 14 days. Cells viability was measured using Alamar Blue assay and apoptosis using human Annexin V-FITC and propidium iodine (PI) kit (BMS306FI; Bender MedSystems, Vienna, Austria). Intracellular Bcl-2, Bax, and caspase-3 levels were measured by flow cytometry. As(2)O(3) significantly reduced CLL cell viability (P < 0.01) and induced apoptotic cell death in a time- and dose-dependent manner. After 7 days, CLL cells showed a significant decrease in mean fluorescence intensity (MFI) of Bcl-2 on flow cytometry study. Bax and caspase-3 levels showed significant decrease in MFI only after prolonged incubations (7 and 14 days) and mostly at higher concentrations of As(2)O(3). The mechanism underlying the reduction in viability of CLL cells incubated with As(2)O(3) is mediated by induction of apoptosis maybe through the down-regulation of Bcl-2. Further studies are needed to elucidate the potential therapeutic role of As(2)O(3) in CLL.