As part of the School Children Have Early Onset of Leading Risk Factors for Cardiovascular Disease and Diabetes Mellitus (SCHOOL) project, this study examines the effect of elevated body mass index on metabolic parameters and its relationship to insulin resistance in prepubertal and postpubertal students from the Wausau School District in central Wisconsin. Two hundred forty-seven nondiabetic students were randomly selected (125 prepubertal [2nd graders] and 122 postpubertal [11th graders]). Waist/hip ratio and body mass index corrected for age and sex were calculated. Fasting insulin, glucose, and nuclear magnetic resonance lipid profiles were measured. Relative insulin resistance was defined as quantitative insulin sensitivity check index > 1 SD below the mean of normal-weight children. Twenty-eight percent of 2nd graders and 33% of 11th graders were overweight. Relative insulin resistance was present in 47% of overweight 2nd graders and 51 % of overweight 11th graders and was associated with higher triglycerides, lower high-density lipoprotein, smaller low-density lipoprotein particles and, in 11th graders, higher waist/hip ratio. Relative insulin resistance prevalence is high among overweight children and adolescents. Biomarkers of increased risk of adverse cardiovascular outcomes are already present in overweight school children.