Fumonisin mycotoxins contaminate maize worldwide. Analysis of maize samples (n = 396) collected from fields in Guatemala from 2000 to 2003 found that lowland maize (<360 m) had significantly more fumonisin B1 than highland maize (>1200 m). For example, 78% of the lowland samples collected at harvest in 2002 contained >0.3 microg/g of fumonisin B1, whereas only 2% of the highland samples contained >0.3 microg/g. Maize from the 2002 crop collected from storage in the highlands just before the 2003 harvest contained significantly more fumonisin B1 compared with levels at harvest in 2002. All Fusarium-infected kernels analyzed from 9 random lowland locations in 2001 were infected with fumonisin-producing Fusarium verticillioides and no other Fusarium species, whereas in samples from the highlands, only 5% of the Fusarium-positive kernels were F. verticillioides. In 2005, maize samples (n = 236) from the 2004 crop were collected from local markets in 20 Departments across Guatemala. The analysis showed that maize from lowland locations was often highly contaminated with fumonisin and was frequently transported to and sold in highland markets. Thus, fumonisin exposure in the highlands will be greatest in groups that obtain their maize in the market place from commercial vendors. Based on a recall study and published consumption data, a preliminary assessment of daily intake of total fumonisins was estimated. Consumption of nixtamalized maize products made from >50% of the maize from commercial vendors in 2005 could result in exposure exceeding the recommended WHO provisional maximal tolerable daily intake.