In the United States, Mexican American children have the highest prevalence of combined overweight and obesity at 43%. Youth obesity prevention interventions have demonstrated limited success. A preliminary trial of an intervention based on social cognitive theory, titled Youth Fit For Life, was conducted with 25 Mexican American children with a mean body mass index (BMI; kg/m2) in the 97th percentile. Measures were BMI for body composition, a shuttle run test for cardiorespiratory fitness, a push-up test for muscular strength, and a reach test for flexibility. Statistically significant improvements in each measure were found over 12 weeks. The effect size for BMI change (d = .14) was similar to findings using the intervention with White (d = .12) and African American (d = .18) children with a mean BMI in the 79th and 85th percentiles, respectively. The need for replication with larger samples and of psychosocial and behavioral factors possibly related to overweight were discussed to better determine the effectiveness of the Youth Fit For Life treatment with Mexican American children. © 2009 Springer Publishing Company.