On the basis of the near-complete linkage disequilibrium of the insulin variable number of tandem repeats (INS VNTR) allele with the neighboring -23Hph1 A/T single-nucleotide polymorphism, previous studies have documented the association of class I ("short") and class III ("long") INS VNTR alleles with metabolic parameters, including circulating insulin levels. Using a new method to sequence class I alleles, we revisited this association in 346 obese children. Class I alleles are made of several types of repeats, whose repartition determines subclasses IC and ID. Fasting insulin was found to be higher in obese children with ID/ID genotypes (135 +/- 12 pmol/l, n = 64) than with ID/IC or IC/IC genotypes (91 +/- 5 pmol/l, n = 97, P = 0.0005). In response to oral glucose, peak insulin levels and insulin-to-glucose area under the curve ratios were higher in ID/ID (872 +/- 122 pmol/l and 109 +/- 15, respectively) than in ID/IC or IC/IC patients (586 +/- 42 pmol/l and 76 +/- 5, P = 0.02 and P = 0.04, respectively). Fasting and postglucose insulin levels were comparable in carriers of IC and of class III alleles. Our results support that the molecular structure of the VNTR allele, not only its overall length, is associated with variations of insulin secretion. ID/ID homozygosity appears responsible for the increased insulin levels previously attributed to the whole class I VNTR group. It will be important to test the ramifications of this observation for class I association with Type 1 (susceptibility) and Type 2 diabetes (protection).