CONTEXT: Polymorphism at the insulin gene (INS) variable number of tandem repeat (VNTR) shows variable associations with childhood body mass index (BMI) in different populations. OBJECTIVE: The objective of this study was to observe INS VNTR associations with body composition and insulin secretion in children. DESIGN: The study was designed as a prospective birth cohort study. PARTICIPANTS: A total of 947 children genotyped for the INS VNTR participated. MAIN OUTCOME MEASURES: Main outcome measures were whole body dual x-ray emission absorptiometry at 9 yr to estimate height-corrected fat mass index (FMI), truncal FMI, and fat-free mass, and insulin secretion after oral glucose at 8 yr. RESULTS: Homozygous III/III children had higher BMI (P = 0.020), FMI (P = 0.015), and truncal FMI (P = 0.022) at 9 yr than class I bearers, but no difference in fat-free mass (P = 0.23). Gain in weight sd score between birth and 3 yr was associated positively with BMI, FMI, and truncal FMI in class I bearers, but not in III/III children (p-interaction with genotype = 0.009-0.066). INS VNTR genotype was not associated overall with insulin secretion at 8 yr (P = 0.64), but class I bearers showed a stronger positive correlation between insulin secretion and BMI at 8 yr (regression coefficient +/- se, 0.26 +/- 0.05; P