['Coronary endothelial dysfunction (ED)-an early marker of atherosclerosis-increases the risk of cardiovascular events.', 'We tested the hypothesis that cholesterol efflux capacity and high-density lipoprotein (HDL) particle concentration predict coronary ED better than HDL-cholesterol (HDL-C).', 'We studied 80 subjects with nonobstructive (<30% stenosis) coronary artery disease. ED was defined as <50% change in coronary blood flow in response to intracoronary infusions of acetylcholine during diagnostic coronary angiography. Cholesterol efflux capacity and HDL particle concentration (HDL-PIMA) were assessed with validated assays. Cholesterol efflux capacity and HDL-PIMA were both strong, inverse predictors of ED (P<0.001 and 0.005, respectively). In contrast, HDL-C and other traditional lipid risk factors did not differ significantly between control and ED subjects. Large HDL particles were markedly decreased in ED subjects (33%; P=0.005). After correction for HDL-C, both efflux capacity and HDL-PIMA remained significant predictors of ED status. HDL-PIMA explained cholesterol efflux capacity more effectively than HDL-C (r=0.54 and 0.36, respectively). The efflux capacities of isolated HDL and serum HDL correlated strongly (r=0.49).'] Do patients With Coronary Endothelial Dysfunction Have Impaired Cholesterol Efflux Capacity and Reduced HDL Particle Concentration?