It was reported that the common −866G/A polymorphism in the promoter of the human uncoupling protein-2 (UCP2) gene, which enhances its trascriptional activity, is associated with increased mRNA levels in human adipocytes and reduced risk of obesity. Studies in knockout mice and β-cells indicate that UCP2 may play a role in β-cell function. In this study, we addressed the question of whether the common −866G/A polymorphism in UCP2 gene contributes to the variation of insulin secretion in humans by genotyping 301 nondiabetic subjects who underwent an oral glucose tolerance test. Glucose-stimulated insulin secretion estimated by several indexes of β-cell function was significantly lower in carriers of the −866A/A genotype compared with −866A/G or −866G/G according to the dosage of the A allele (P = 0.002–0.05). To investigate directly whether the UCP2 −866G/A polymorphism affects human islet function, pancreatic islets isolated from two −866G/G homozygous, seven −866G/A heterozygous, and one −866A/A homozygous nondiabetic donors were studied. Islets from −866A/A homozygous had lower insulin secretion in response to glucose stimulation as compared with −866G/G and −866G/A carriers. These results indicate that the common −866G/A polymorphism in the UCP2 gene may contribute to the biological variation of insulin secretion in humans.