OBJECTIVE: To investigate differences in associations between physical activity and over-weight for students in two adjacent areas on the border between Mexico and the United States of America: students in the city of Matamoros, Mexico, and Mexican-American students in the Lower Rio Grande Valley (LRGV) area of southern Texas. Since the extremely high prevalence of overweight among Mexican-American adolescents is well-recognized, we wanted to determine whether overweight has become a problem among Mexican adolescents as well. METHODS: Students from 6 schools (n = 653), representing 11% of the ninth-grade students in Matamoros during 2002-2003, and students from 13 high schools (n = 4 736), representing 22% of the ninth-grade students in the LRGV during 2000-2001, completed questionnaires. Polytomous logistic regression was performed to estimate the risk of being at risk for overweight (> 85th percentile to < 95th percentile of body mass index (BMI) for age and sex) and the risk of being overweight (> 95th percentile of BMI-for-age and sex) versus normal weight that were associated with measures of physical activity. For simplicity, the classification of normal weight also included underweight. RESULTS: A higher percentage of adolescents in the LRGV were at risk of overweight (17%) in comparison with adolescents from Matamoros (15%). The percentages of LRGV and Matamoros adolescents who were overweight were identical (17%). LRGV adolescent boys (OR = 0.87, 95% CI = 0.77-0.98) who participated in team sports were less likely to be at or above the 85th percentile of BMI-for-age and sex. Although of borderline significance, Matamoros and LRGV adolescent boys who participated in physical education classes were less likely to be at risk for overweight. Among neither the Matamoros students nor the LRGV students were any of the various other physical activity categories or levels associated with being at risk for overweight or being overweight. CONCLUSIONS: Nearly one-third of the students in both Matamoros and the LRGV are at risk for overweight or are overweight. Implementation of interventions on healthful dietary choices and participation in physical education classes and sports teams are essential for reducing the extremely high prevalence of overweight among students on both sides of the Texas/Mexico border.