The high-affinity sulfonylurea receptor (SUR1) gene regulates insulin secretion and may play a role in type 2 diabetes. A silent variant in exon 31 of SUR1 (AGG→AGA) was detected by single-strand conformational polymorphism and genotypes were determined for 396 Mexican American subjects (289 non-diabetic). The normal and mutant alleles were designated G and A, respectively. Among non-diabetics, those with the AA genotype had higher fasting insulin values than those with the AG and GG genotypes (113.4 pmol/l for AA vs 82.8 pmol/l for AG/GG, P=0.043). Similar results were observed for 2-h insulin (849.6 pmοl/l for AA vs 498.6 pmol/l for AG/GG, P=0.0003) and for the proinsulin to specific insulin ratio (0.068 for AA vs 0.056 for AG/GG, P=0.030). Specific insulin levels also differed significantly across the three genotypic classes (P=0.021). No differences in fasting glucose, body mass index, or waist circumference according to genotype were noted. Two-hour glucose was modestly higher in individuals with the AA genotype. Since we have previously reported linkage between SUR1 and hyperglycemia, the present association between a SUR1 variant and hyperinsulinemia in normal individuals from a high diabetes risk ethnic group raises the possibility of primary insulin hypersecretion as an antecedent of type 2 diabetes in at least some individuals from this population.