We conducted a cross-sectional study among 1,717 children and adolescents of Mexican origin ages 5-19 years living in Mexico and Texas to explore the influence of country of birth and country of longest residence on their overweight and obesity status. Descriptive statistics were used to compare demographic and anthropometric characteristics of participants born and raised in Mexico (Mexicans), born in Mexico and raised in the United States (Mexican immigrants), and born and raised in the United States (Mexican-Americans). Univariate and multivariate nominal logistic regression was used to determine the demographic predictors of obesity adjusted by country of birth, country of residence, age, and gender. Almost half (48.8%) of the Mexican-Americans and 43.2% of the Mexican immigrants had body mass index at the 85th percentile or above, compared to only 29.3% of the Mexicans (P < .001). Thus, Mexican-Americans and Mexican immigrants were more likely to be obese than their Mexican peers [Mexican-Americans: odds ratio (OR) = 2.5 (95% confidence interval [CI] 1.8-3.4); Mexican immigrants: OR = 2.2 (95% CI 1.6-3.0)]. In addition, males were more likely than females to be obese [OR = 1.6 (95% CI 1.2-2.1)], and adolescents 15-19 years of age were less likely than their younger counterparts [OR = 0.5 (95% CI 0.4-0.7)] to be obese. The high prevalence of obesity among children of Mexican origin in the United States is of great concern and underscores the urgent need to develop and implement obesity preventive interventions targeting younger children of Mexican origin, especially newly arrived immigrant children. In addition, future obesity research should take into consideration the country of origin of the study population to develop more culturally specific obesity interventions.