Arsenic trioxide (As₂O₃) is one of the arsenic compounds found in nature. As₂O₃ has recently been used to treat patients suffering from retinoic acid receptor (AML). It is of clinical interest to investigate whether As₂O₃ is also effective in treating solid tumors. Here, we report that As₂O₃ exhibited inhibitory effects on the proliferation of human breast cancer MCF‐7 cells in a dose‐ and time‐dependent manner. The 50% inhibitory concentration (IC₅₀) of As₂O₃ in inhibiting proliferation of MCF‐7 cells were 8, 1.8, and 1.2 μM upon 1‐, 2‐, and 3‐day treatment, respectively. In elucidating the underlying action mechanisms, the results of experiments concerning DNA fragmentation and externalization indicated that As₂O₃ exerted its action on MCF‐7 cells via apoptosis, whereas the result of flow cytometry also indicated that As₂O₃ could induce mitochondrial mediated cell‐cycle arrest at G₁ phase. Further studies by Western blot analysis indicated that As₂O₃ regulated apoptosis and the expression of cell‐cycle‐related proteins as it upregulated p53 protein level and downregulated bcl‐2 protein level. Results in present study indicated that As₂O₃ might also be a good candidate for treating breast cancer. © 2004 Wiley‐Liss, Inc.