Vitamin C is an essential nutrient with several different functions in the human body. Humans and a few other animal species are incapable of producing it, and therefore depend on a balanced diet to ensure adequate plasma levels of this nutrient. The severe deficiency of vitamin C leads to scurvy, and a balanced diet alone can effectively prevent scurvy, but cannot guarantee a good health condition. This led to the conclusion that to be in good health man needs amounts of vitamin C in the order of grams (mega-doses) per day. Mega doses of vitamin C administered through intravenous injection can produce high plasma concentrations that in vitro show a powerful cytotoxic effect on a number of different human tumour cell lines. A number of different mechanisms have been advocated to explain the selective cytotoxicity of vitamin C to cancer cells, but whatever the mechanism may be, this observation has the potential for revolutionising the field of cancer chemotherapy.