In the present study, we tested the hypothesis that age-associated vascular endothelial dysfunction is exacerbated by IFG (impaired fasting plasma glucose) and that regular aerobic exercise prevents this effect. Data were analysed from a cohort of 131 non-smoking men and women without overt clinical disease. Compared with young adult controls (age=24±1 years, n=29; values are means±S.E.M.), brachial artery FMD (flow-mediated dilation), a measure of conduit artery EDD (endothelium-dependent dilation), was 33% lower [7.93±0.33 against 5.27±0.37%Δ (% change), P<0.05] in MA/O (middle-aged/older) adults with NFG (normal fasting plasma glucose) (≤99 mg/dl, 62±1 years, n=35). In MA/O adults with IFG (100-125 mg/dl, 64±1 years, n=28), FMD was 30% lower (3.37±0.35%Δ) than in their peers with NFG and 58% lower than young controls (P<0.05). Brachial artery FMD was greater (6.38±0.35%Δ) in MA/O adults with NFG who regularly performed aerobic exercise (>45 min/day for ≥5 days/week, 62±1 years, n=23) compared with their non-exercising peers and only slightly less than young controls (P<0.05). Most importantly, FMD was completely preserved in MA/O adults with IFG who regularly performed aerobic exercise (6.99±0.69%Δ, 65±1 years, n=16). In the pooled sample, fasting plasma glucose was inversely related to FMD (r=-0.42, P<0.01) and was the strongest independent predictor of FMD (R(2)=0.32). Group differences in FMD were not affected by other subject characteristics or brachial artery properties, including brachial artery dilation to sublingual NTG (nitroglycerine, i.e. endothelium-independent dilation). IFG exacerbates age-associated vascular endothelial dysfunction and this adverse effect is completely prevented in MA/O adults who regularly perform aerobic exercise.