The American Cancer Society estimates 1,368,050 new cancer cases in 2004 and 536,700 deaths from the disease. Although cancer rates for different sites vary widely, the 5-year relative survival rate for all cancers combined is estimated at 63%. Among the many possible causes of cancer, reactive oxygen species (ROS) have been implicated in the onset and development of the disease. Experimental studies support this notion in part by showing that antioxidants that prevent ROS damage can act as cancer preventive agents. Once cancer has occurred, however, radiation therapy and some forms of chemotherapy rely on ROS toxicity to eradicate tumor cells, which raises the question of whether antioxidants help or hinder the overall outcome of cancer treatment.