Background Differences in the prevalence of youth at or above the 85th percentile of age- and gender-adjusted body mass index (BMI) by race/ethnicity and socioeconomic status were examined among youth in 8th and 10th grades. The possible role of a number of lifestyle behaviors and family/parenting factors in explaining these differences was then explored. Methods Cross-sectional survey data were used from nationally representative samples in the Monitoring the Future study from 1998 to 2003 (N=39,011 students). Data were analyzed in 2006. Results Minority, low-income males, and male youth were more likely have a BMI at or above the 85th percentile. Frequency of eating breakfast, eating fruits and vegetables, and exercising regularly were inversely associated with being at or above the 85th percentile. The number of hours youth spend per week watching television was positively associated with being at or above the 85th percentile. These lifestyle behaviors proved more important than the family/parenting variables examined. Conclusions The overrepresentation of youth at risk of overweight or overweight among racial/ethnic minority and low-income populations mimics the excess morbidity of overweight and obesity-related health conditions in these same populations. Differences in lifestyle behaviors and family characteristics might help to explain these subgroup differences starting at an early age. While there is growing need to modify these behaviors in the population at large, the need is greatest among minorities and low-socioeconomic status youth.