Objective: This study sought to test whether patients with early-onset coronary artery disease (EOCAD, n=30) showed systemic microvascular rarefaction and endothelial dysfunction in comparison to age- and sex-matched healthy controls (CTL, n = 30), as evaluated by skin video-capillaroscopy. Methods: Functional capillary density (FCD)was defined as the number of spontaneously perfused capillaries per square millimeter of skin area and assessed by high-resolution intra-vital color microscopy in the dorsumof the middle phalanx. Capillary recruitment (capillary reserve)was evaluated using post-occlusive reactive hyperemia (PORH) after arm ischemia for 3 min. Results: Themean capillary density at restwas significantly reduced in patientswith EOCADcompared to controls (CTL 95±20 and EOCAD 80±18 capillaries/mm2, P= 0.0040). During PORH, capillary density was alsomarkedly reduced in EOCAD patients (CTL 96 ± 18 and EOCAD 71 ± 20 capillaries/mm2, P b 0.0001). Moreover, the capillary density in EOCAD patients was significantly reduced during PORH (EOCAD at rest 80± 19 and EOCAD during PORH 71 ± 20 capillaries/mm2, P = 0.0073). Conclusions: Patientswith EOCAD presented systemic capillary rarefaction and impairedmicrovascular endothelial function. Thus, the early detection of these microvascular alterations in young adults at an increased risk of coronary artery disease could be useful as a surrogate marker of subclinical atherosclerosis.Jan. 2016