Context: IGF-I and its binding proteins influence growth, development, and disease risk. Studies have revealed ethnic variations in the IGF system. Objective: This longitudinal studywasundertaken to test the hypothesis that the ethnic differences in the IGF system exist throughout the pubertal transition, and these differences are mediated at least in part by inherent differences in insulin dynamics. Design: This was a longitudinal study. Annual evaluations were conducted for pubertal maturation, body composition, acute insulin response to glucose (AIRg), and reproductive-endocrine profile. Hormones and binding proteins were determined using standard assays, the AIRg during a frequently sampled iv glucose tolerance test, and body composition by dual-energy x-ray absorptiometry. Mixed model analyses were used to identify and characterize ethnic differences in the IGF system across the pubertal transition after adjusting for ethnicity, sex, age, maturation status, body composition, and reproductive hormones, and to identify the contribution of insulin to IGF binding protein (IGFBP)-1. Participants: Subjects included African-American (AA) and European American children (n = 162 at baseline) aged 7-16 yr, evaluated across the pubertal transition. Main Outcome Measures: Annual data on IGF-I, IGFBP-1, and IGFBP-3 were examined. Results: IGF-I was higher in AA children at pubertal stage 1 only (P < 0.001). However, IGFBP-3 and IGFBP-1 concentrations were lower in AAs through much of puberty (P < 0.05). The lower IGFBP-1 of AAs was in part explained by greater AIRg. Conclusions: Our data suggest that the higher IGF-I and lower IGFBP-1 and IGFBP-3 levels in AAs as compared with European Americans during puberty suggest potential ethnic differences in circulating bioavailable IGF-I. In addition, higher AIRg in AAsmaylead to greater bioavailable IGF-I. Whether these differences in the IGF system account for disparities in disease risk warrants further investigation. Copyright © 2008 by The Endocrine Society.