A review is presented of the epidemiology of vitamin C. This review summarizes the epidemiologic relationship of vitamin C intake to mortality from all causes, cancer, and cardiovascular diseases and the results of randomized controlled trials involving vitamin C supplements and mortality. Many studies show an inverse relationship between mortality and vitamin C intake, particularly for levels close to the current U.S. recommended dietary allowance (RDA) of 75-90. mg per day for adults. The inverse relationship is particularly strong for serum vitamin C. However, several studies show no relationship at all or no significant relationship after controlling for confounding variables. There is little evidence of a relationship at high or low levels of vitamin C supplement intake. The results of randomized intervention trials assessing the impact of supplemental vitamin C on mortality and cancer survival are largely negative, but there are selective instances of benefits. There have been very few intervention trials focusing on vitamin C. Most of the available prospective cohort data on vitamin C and mortality have not been analyzed. Additional cohort analyses and new intervention trials would substantially refine the evidence. Vitamin C is an essential human nutrient and there is substantial epidemiologic evidence that regular intake of at least the RDA of vitamin C is beneficial for optimum health.