OBJECTIVE: To investigate the anticancer effects of rutin on human colon cancer LoVo cells and breast cancer MCF-7 cells. STUDY DESIGN: The anticancer effects of rutin were measured through the process of cell viability assay, cytotoxicity assessment by lactate dehydrogenase (LDH) assay, cell cycle analysis by flow cytometry, cell apoptosis assessment, and intracellular H2O2 and O2 •- production measurement. RESULTS: Rutin inhibited the proliferation of LoVo and MCF-7 cancer cells in a dose-dependent manner, with the IC50 values of 29.1 μM for LoVo cells and 45.6 μM for MCF-7 cells, respectively. Furthermore, flow cytometric assay revealed that rutin could mediate the cell-cycle arrest principally in the S phase after 24 hours of treatment with the two tumor cells. Assessment of cell apoptosis revealed that 52.8% of LoVo cells and 70.8% of MCF-7 cells entered the early phase of apoptosis when treated with 100 μM rutin for 48 hours. Moreover, we also found the generation of reactive oxygen species (ROS) is a critical mediator in rutininduced cell apoptosis. CONCLUSION: Rutin displays high antitumor activity via a ROS-dependent apoptosis pathway and has the great potential to be used as an antitumor precursor compound. © Science Printers and Publishers, Inc.