Our aim was to examine the effects of food acculturation on Mexican Americans' (MA) diets, taking the Mexican diet as reference. We used nationally representative samples of children (2-11 y) and female adolescents and adults (12-49 y) from the Mexican National Nutrition Survey 1999 and NHANES 1999-2006 to compare the diets of Mexicans (n = 5678), MA born in Mexico (MAMX) (n = 1488), MA born in the United States (MAUS) (n = 3654), and non-Hispanic white Americans (NH-White) (n = 5473). One 24-h diet recall was used to examine the percentage consuming and percentage energy consumed from selected food groups. Most of the food groups analyzed displayed a fairly linear increase or decrease in percent energy/capita intake in this order: Mexican, MAMX, MAUS, NH-White. However, few significant differences were observed among the US subpopulations, especially among MAUS and NH-Whites. Overall, compared to Mexicans, the US subpopulations had greater intakes of saturated fat, sugar, dessert and salty snacks, pizza and French fries, low-fat meat and fish, high-fiber bread, and low-fat milk, as well as decreased intakes of corn tortillas, low-fiber bread, high-fat milk, and Mexican fast food. Furthermore, the patterns were similar in all age groups. Although we found a mix of positive and negative aspects of food acculturation, the overall proportion of energy obtained from unhealthy foods was higher among the US subpopulations. Our findings indicate that within one generation in the US, the influence of the Mexican diet is almost lost. In addition, our results reinforce the need to discourage critical unhealthful components of the American diet among MA.