Atrial fibrillation is associated with alterations in HDL function, metabolism, and particle number Abstract Increased morbidity and mortality in atrial fibrillation (AF) are related to the pro-fibrotic, pro-thrombotic, and pro-inflammatory processes that underpin the disease. High-density lipoproteins (HDL) have anti-inflammatory, anti-oxidative, and anti-thrombotic properties. Functional impairment of HDL may, therefore, associate with AF initiation or progression. We studied indices of HDL quality and quantity of AF patients and healthy controls, including HDL-particle number, HDL cholesterol, apolipoprotein (apo) A–I levels, serum amyloid A (SAA) content and HDL-cholesterol efflux capacity, and paraoxonase activity of apoB-depleted serum. Serum samples were collected from AF patients (n = 91) before catheter ablation and from age- and sex-matched control subjects (n = 54). HDL-cholesterol efflux capacity was assessed in a validated assay using [3H]-cholesterol-labeled J774 macrophages. Lecithin–cholesterol acyltransferase (LCAT) and paraoxonase activities were assessed using fluorometric assays, SAA levels were determined by ELISA, and total and subclass HDL-particle number was assessed by nuclear magnetic resonance spectroscopy. ApoA-I levels were determined by immunoturbidimetry. HDL-cholesterol efflux capacity, HDL-particle number, apoA-I levels, and LCAT activity were markedly reduced in AF patients when compared to healthy individuals (all p < 0.001), whereas HDL-associated paraoxonase activity and SAA content were not altered (p = 0.578, p = 0.681). Notably, cholesterol efflux capacity, HDL-particle number, apoA-I levels as well as LCAT activity recovered following restoration of sinus rhythm (all p < 0.001). We identified marked alterations in HDL function, HDL maturation, and HDL-particle number in AF patients. Assessing HDL-particle number and function in AF may be used as a surrogate marker of AF onset and progression and may help identifying patients at high risk.