Obstructive sleep apnea (OSA) is highly prevalent in patients with established coronary artery disease. 1 We have reported that OSA is also a common comorbidity, affecting approximately two-thirds of patients who had a recent myocardial infarction (MI). 2 However, the prognostic implications of OSA after an MI remain unknown.Endothelial dysfunction is an early marker of vascular function impairment and is also predictive of future cardiovascular events. 3 Measurement of fl owmediated dilation (FMD ) is recognized as a measure of endothelial dysfunction and has been used to determine risk factors for cardiovascular disease in several clinical studies. 4 Moreover, FMD is closely related to coronary endothelial function. 5 Prior studies have shown diffuse endothelial dysfunction in patients with long-term 6 and short-term coronary syndromes. 7 Epidemiologic and experimental data suggest that patients with OSA have impaired endothelial function, a mechanism that may help explain the association between OSA and cardiovascular diseases. 8,9 However, to our knowledge, there Background: Impaired brachial fl ow-mediated dilation (FMD) is associated with risk for subsequent cardiovascular events in patients after myocardial infarction (MI). These patients often have obstructive sleep apnea (OSA). We tested the hypothesis that patients with OSA post MI will exhibit more severe impairment in FMD. Methods: We studied 64 patients with MI admitted to our hospital. OSA was determined using polysomnography. FMD was measured using high-resolution ultrasonography, with researchers blind to the OSA diagnosis.