Cancer is the first leading cause of death in Korea and the second leading cause of death in the USA. There is extensive research into prevention of cancer and the support of oncology patients with diet or dietary supplements. In vitro and in vivo animal studies have indicated that antioxidants, including beta-carotene, alpha-tocopherol, and ascorbic acid, can yield anti-cancer effects in addition to providing protection against oxidative damage. Although many observational studies have shown that consuming fruits and vegetables can reduce the risk of some cancers, the results of several large-scale human intervention trials testing the benefits of a single or combined higher-dose of individual micronutrients have been inconsistent. Cancer can cause profound metabolic and physiological changes which may affect patients' nutrient requirements. Although the optimal route of nutrient delivery is through diet, cancer patients often suffer symptoms that disrupt their food intake, including anorexia, premature satiety, altered taste and smell, and changes in bowel mobility. In particular, micronutrient deficits can slow postoperative healing, contribute to depression symptoms, and decrease immune competence. Cancer patients are generally motivated to take dietary supplements to improve responses to treatment and quality of life. The Physician's Health Study II (PHS II) randomized controlled trial reported recently that daily multivitamin supplementation significantly, albeit modestly, reduced the risk of total cancer. Although evidence of multivitamin use benefits is limited in cancer patients, taking dietary supplements with constituents in the range of the recommended daily allowance according to the Dietary Reference Intake (DRI) recommendation is generally considered to be safe.