Mutations of the hepatic nuclear factor-1alpha (HNF-1alpha) gene have been found in patients with maturity-onset diabetes of the young. We examined the relation between the I27L polymorphism of HNF-1alpha and insulin sensitivity and beta-cell function assessed by a hyperglycemic clamp. This study included 52 healthy glucose-tolerant and normotensive subjects (age, 19-40 yr; body mass index, 17.58-35.61 kg/m2; waist/hip ratio, 0.65-1.03). We identified 19 LL subjects, 24 IL, and 9 II subjects. No difference was noted in the demographic features among the three genotypes. The LL group had the highest postchallenge insulin levels at 30 and 90 min (P = 0.038 and P = 0.015, respectively) and also the highest insulin area under curve (P = 0.009) among the three genotypes. The LL group was more insulin resistant than the IL and II groups (P = 0.042 for insulin sensitivity index). After adjusting for age, gender, obesity, and ethnicity, the I27L polymorphism was an independent determinant of the insulin sensitivity index (P = 0.001). However, it had no impact on either the first or second phase insulin response. Therefore, we conclude that the I27L polymorphism is associated with insulin resistance, but not beta-cell function. The mechanism of this association is unclear, but HNF-1alpha may play a role in regulating hepatic glucose metabolism.