Obesity has reached epidemic proportions, in the United States as well as among its trade partners such as Mexico. It has been established that an "obesogenic" (obesity-causing) food environment is one influence on obesity prevalence. To isolate the particular role of NAFTA, the North American Free Trade Agreement, in changing Mexico's food environment, we plotted the flow of several key products between the United States and Mexico over the 14-year NAFTA period (1994-2008) and situated them in a broader historical context. Key sources of USDA data include the Foreign Agricultural Service's Global Agricultural Trade System, its official repository for current and historical data on imports, exports and re-exports, and its Production, Supply, and Distribution online database. US export data were queried for agricultural products linked to shifting diet patterns including: corn, soybeans, sugar and sweeteners, consumer-oriented products, and livestock products. The Bureau of Economic Analysis' Balance of Payments and Direct Investment Position Data in their web-based International Economic Accounts system also helped determine changes in US direct investment abroad from 1982 to 2009. Directly and indirectly, the United States has exported increasing amounts of corn, soybeans, sugar, snack foods, and meat products into Mexico over the last two decades. Facilitated by NAFTA, these exports are one important way in which US agriculture and trade policy influences Mexico's food system. Because of significant US agribusiness investment in Mexico across the full spectrum of the latter's food supply chain, from production and processing to distribution and retail, the Mexican food system increasingly looks like the industrialized food system of the United States.