Recent evidence suggests that mitochondrial uncoupling protein 2 (UCP2) in pancreatic beta-cells plays a crucial role in insulin production and secretion. We hypothesized that 2 UCP2 polymorphisms, a -55C/T (Ala55Val) substitution in exon 4 and an exon 8 insertion, would alter the acute insulin response to glucose (AIRg). Subjects were 155 African American (AA) and European American (EA) women. Body composition was determined by dual-energy x-ray absorptiometry. Insulin sensitivity and AIRg were measured with an intravenous glucose tolerance test and minimal modeling. To account for the confounding effects of population stratification, estimates of African admixture were obtained from approximately 35 ancestry-informative markers. Uncoupling protein 2 genotyping was conducted with gel electrophoresis. Information was analyzed using mixed linear models. A positive association between the -55C/T homozygous mutation and AIRg was identified in the total sample (P < .01) and independently in EA women (P = .02) but not AA women. The exon 8 insertion did not significantly affect AIRg. No interaction effects of the 2 polymorphisms on AIRg were noted. These results indicate that AIRg is associated with the -55C/T UCP2 homozygous mutation and that the presence of this mutation could alter postchallenge insulin concentration.