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To determine whether the peroxisome proliferator–activated receptor (PPAR)-γ Pro12ala polymorphism modulates susceptibility to diabetes in South Asians.South Asians (n = 697) and Caucasians (n = 457) living in Dallas/Forth Worth, Texas, and South Asians living in Chennai, India (n = 1,619), were enrolled for this study. PPAR-γ Pro12Ala was determined using restriction fragment–length polymorphism. Insulin responsiveness to an oral glucose tolerance test (OGTT) was measured in nondiabetic subjects.The Caucasian diabetic subjects had significantly lower prevalence of PPAR-γ 12Ala when compared with the Caucasian nondiabetic subjects (20 vs. 9%, P = 0.006). However, there were no significant differences between diabetic and nondiabetic subjects with reference to the Pro12Ala polymorphism among the South Asians living in Dallas (20 vs. 23%) and in India (19 vs. 19.3%). Although Caucasians carrying PPAR-γ Pro12Ala had lower plasma insulin levels at 2 h of OGTT than the wild-type (Pro/Pro) carriers (76 ± 68 and 54 ± 33 μU/ml, respectively, P = 0.01), no differences in either fasting or 2-h plasma insulin concentrations were found between South Asians carrying the PPAR-γ Pro12Ala polymorphism and those with the wild-type genotype at either Chennai or Dallas.Although further replication studies are necessary to test the validity of the described genotype-phenotype relationship, our study supports the hypothesis that the PPAR-γ Pro12Ala polymorphism is protective against diabetes in Caucasians but not in South Asians.