To determine whether endothelial dysfunction precedes the clinical diagnosis of diabetes mellitus, we investigated the relation of endothelial flow-mediated dilation (FMD) with fasting plasma glucose among a multiethnic population-based cohort of 579 nondiabetic subjects without previous myocardial infarction or stroke enrolled in the Northern Manhattan Study (age 66 +/- 9 years; 41% men, 16% white, 15% black, and 68% Hispanic). Impaired fasting glucose or prediabetic status, defined as a fasting glucose level of 100 to 125 mg/dl, was present in 95 subjects (16%). Endothelial function was determined using FMD during reactive hyperemia. Multiple linear regression analyses were used to assess the relation between plasma glucose and endothelial function after adjustment for potential confounders. FMD was significantly lower (4.9 +/- 3.8% vs 6.1 +/- 3.7%, p = 0.003) in those with impaired fasting glucose than in subjects with normal fasting glucose. Prediabetic status was significantly associated with impaired FMD (odds ratio 1.9, 95% confidence interval 1.1 to 3.1, p = 0.02). After adjustment for age, gender, body mass index, and hypertensive status, a higher fasting glucose was significantly associated with a lower FMD (beta = -0.024 +/- 0.012, p = 0.04) in a continuous linear relation. Thus, for each 10-mg/dl increase in plasma glucose, a 0.24% decrease occurred in FMD. Impaired FMD was present among prediabetics. An elevated fasting plasma glucose level is associated with impaired endothelial function among nondiabetics. These results further support the role of hyperglycemia in the pathogenesis of vascular dysfunction at different stages of diabetes development and the role of impaired fasting glucose as a risk factor for macrovascular disease.