Objectives <p>High cardiorespiratory fitness is a vital part of cardiovascular prevention in health and disease. However, the optimal age of aerobic exercise interventions is unclear and animal studies suggest a blunted effect of training on cardiorespiratory fitness with a priori endothelial dysfunction. This study aims to assess the effect of pre-existing endothelial dysfunction on the association between age and cardiorespiratory fitness as well as the change of the latter in response to 8 weeks of aerobic exercise training.</p> Methods <p>Cardiorespiratory fitness (peak oxygen consumption [<InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\dot{V}\)</EquationSource> </InlineEquation>O_2peak]), brachial-arterial flow-mediated dilation (baFMD) and cardiovascular risk were assessed in 164 apparently healthy disease-free middle-aged adults with and without pre-existing endothelial dysfunction and cardiovascular risk factors (<i>n</i> = 36 arterial hypertension,<i> n</i> = 59 obesity,<i> n</i> = 118 dyslipidemia,<i> n</i> = 15 diabetes mellitus type II,<i> n</i> = 30 chronic inflammation). 30 of them underwent a randomized-controlled and supervised 8-week aerobic training intervention followed by a second assessment of <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(\dot{V}\)</EquationSource> </InlineEquation>O_2peak, baFMD and cardiovascular risk. Multiple linear regression models were used to address the study aims.</p> Results <p>No clear evidence was found for a moderating effect of baFMD on the relationship of <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(\dot{V}\)</EquationSource> </InlineEquation>O_2peak with age (F(3, 151) = 1.37, <i>p = 0.254</i>). The improvement of <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(\dot{V}\)</EquationSource> </InlineEquation>O_2peak by aerobic exercise was significantly attenuated with lower baseline baFMD (F(3, 5.33) = 43.13, df = 3,<i> p &lt; 0.001</i>).</p> Conclusions <p>The results provide evidence in middle-aged adults for an attenuated response of cardiorespiratory fitness to aerobic exercise in the presence of endothelial dysfunction, advocating for the early involvement of aerobic exercise within primary prevention of cardiovascular diseases and low cardiorespiratory fitness.</p> Trial Registration <p>TRN: (COmPLETE Health cohort [NCT03986892] and VascuFit cohort [NCT05235958]).</p>