There is growing interest in the potential for folic acid fortification in the Americas and recognition of the high prevalence of low plasma vitamin B12 concentrations reported in various studies. This review summarized available data on plasma vitamin B12 and folate concentrations in the Americas. At least 40% of individuals had deficient or marginal plasma vitamin B12 concentrations in almost all locations and across age groups. Low plasma folate concentrations were less common. It is hypothesized that vitamin B12 deficiency may result from a low intake of animal source foods, while a higher intake of refined flour may result in low plasma folate.