OBJECTIVE Earlier we found elevated insulin levels in obese children and adolescents. The present study examines whether alterations in insulin secretion and/or clearance contribute to hyperinsulinemia in obese adolescents. METHODS Fasting circulating insulin and C-peptide concentrations were examined in 1157 adolescents, aged 11-18 y, from a biracial (black/white) community. In this epidemiologic study, plasma C-peptide was used as a noninvasive measure of insulin secretion by beta cells, C-peptide to insulin ratio as an indicator of hepatic insulin extraction, and insulin to glucose ratio as a measure of insulin sensitivity. Body mass index (BMI) was used as an index of obesity, since it is strongly associated with insulin levels and the C-peptide to insulin ratio more so than with measures of skinfolds and percent body fatness. RESULTS Obese individuals (BMI > 90th P) had higher levels of plasma insulin (23.7 mu/ml vs 11.7 mu/ml), C-peptide (2.7 ng/ml vs 1.7 ng/ml), and insulin to glucose ratio (0.29 vs 0.15), and lower C-peptide to insulin ratio (0.13 vs 0.16) than non-obese adolescents (all P < 0.001). Elevated C-peptide and decreased C-peptide to insulin ratio were noted in subjects with both obesity and hyperinsulinemia (insulin > 90th P) versus those without these conditions (P < 0.001). Individuals with obesity and low insulin clearance (C-peptide/insulin < 10th P) had 18-fold higher prevalence of hyperinsulinemia versus those without these conditions. Although black adolescents, despite their lower percent body fat, had higher insulin and lower C-peptide and C-peptide to insulin ratio than their white counterparts, BMI related positively to insulin and C-peptide, and inversely with C-peptide to insulin ratio in both races. CONCLUSIONS These data suggest that both increased insulin secretion and decreased insulin clearance contribute to hyperinsulinema in obese adolescents.