Abstract: OBJECTIVE To investigate whether the apoptotic volume decrease is the early event in arsenic trioxide induced apoptosis in breast cancer MCF-7 cells. METHODS The effect of As₂O₃ on cell viability was assayed by methylthiazoletetrazolium(MTT). Apoptosis was analyzed by flow cytometry. Lag-time microphotography was employed to detect cell volume. The apoptotic signaling activity of Caspase-3 was detected by caspase assay kit. RESULTS MTT result showed As₂O₃ reduced the viability of MCF-7 cells in a concentration-dependent manner. Flow cytometry assay confirmed apoptosis resulted in As₂O₃-treated MCF-7 cells. Apoptotic volume decrease was observed within 2 h in As₂O₃ treated MCF-7 cells, but the caspase-3 activity had no significant change. The caspase-3 activity increased significantly until As₂O₃ treated MCF-7 cells for 6 h. CONCLUSION Apoptotic volume decrease is the early event in arsenic trioxide induced apoptosis in breast cancer cells.