Purpose: The purpose of this study was to investigate the change of fasting proinsulin and insulin in overweight/obese prepubertal children with normal glucose tolerance (NGT) compared to lean children. Subjects and Methods: A total of 109 prepubertal children (mean 8.6 yrs) consisting of lean children (n=52) and overweight (n=14) or obese (n=43) children with NGT were included. Insulin resistance was estimated using the homeostasis model assessment of insulin resistance (HOMA – IR). Proinsulin and insulin were measured by radioimmunoassay and immunoradiometric assay, respectively, which did not cross-react each other. Results: Fasting proinsulin progressively increased as the degree of obesity increased between lean (7.22±3.01 pM/L), overweight (12.31±2.91 pM/L) and obese (12.31±2.91 pM/L) children, controlling for age, gender, family history of type 2 diabetes mellitus (T2DM) and HOMA-IR (P=0.001). However, no significant difference in the ratios of fasting proinsulin to insulin between the 3 groups was observed. The BMI z-score (r=0.33, β=1.289, P=0.001) and HOMA-IR (r=0.57, β=2.628, P＜0.0001) had independent effects on fasting proinsulin, regardless of age, gender and family history of T2DM. The correlation coefficients between fasting proinsulin and HOMA-IR in children with FBS ≥ 89 mg/dL (n=25) was higher than that in those with FBS ＜ 89 mg/dL (n=84). Conclusion: It is concluded that obesity and IR may independently bring about fasting hyperproinsulinemia in prepubertal children, which is proportional to hyperinsulinemia as far as NGT is preserved. Fasting proinsulin reflects IR in prepubertal children, especially in those with high normal fasting glucose.