BACKGROUND: Treatment of the metabolic syndrome in adults is generally approached with diet and physical activity. The influence of diet and physical activity on cardiometabolic outcomes in children has not been clearly established. OBJECTIVE: The main objective of this study was to test the hypothesis that the distribution of energy from fat and carbohydrate in addition to limited time spent engaging in physical activity would contribute to the prevalence of the metabolic syndrome and its components in a multiethnic pediatric population. DESIGN: Observational, cross-sectional study. Diet was assessed by two 24-hour recalls, physical activity by accelerometry, body composition by dual-energy absorptiometry, and glucose and lipid levels using fasting sera. MAIN OUTCOME MEASURES: Presence of metabolic syndrome and its components. SUBJECTS: 202 African-American (n=79), white (n=68), or Hispanic (n=55) healthy children aged 7 to 12 years. STATISTICAL ANALYSIS: The contribution of diet and physical activity to the metabolic syndrome and its components were assessed by logistic regression and multiple linear regression analyses. RESULTS: Prevalence of the metabolic syndrome in the total sample was 8.4%, with Hispanics more likely than whites and African Americans to meet the criteria. A greater intake of energy from carbohydrate was related to a greater waist circumference and higher concentrations of triglyceride and glucose particularly apparent within the African-American sample (P<0.05). Fat intake was associated with a lower waist circumference (P<0.05) and with lower concentrations of triglyceride (P<0.05) and glucose (P<0.001) in the total sample. Greater moderate/hard physical activity was associated with higher high-density lipoprotein cholesterol concentrations in whites (P<0.05). Increased sedentary behavior was related to greater glucose concentration in whites and Hispanics (P<0.05 for both). CONCLUSIONS: Diet composition was more closely related to the components of the metabolic syndrome than was physical activity, with carbohydrate intake being adversely related to waist circumference, triglyceride levels, and glucose levels. Furthermore, relationships among diet and metabolic syndrome outcomes were stronger among African-American children, suggesting that nutrition interventions in this group may be particularly beneficial.