ObjectiveTrimethylamine-N-oxide (TMAO) is a metabolite derived from the microbial processing of dietary phosphatidylcholine and carnitine and the subsequent hepatic oxidation. Due to its prothrombotic and inflammatory mechanisms, we aimed to assess its role in the prediction of adverse events in a susceptible population, namely patients with atrial fibrillation.MethodsBaseline TMAO plasma levels were measured by liquid chromatography-tandem mass spectrometry in 2379 subjects from the ongoing Swiss Atrial Fibrillation cohort. 1722 underwent brain MRI at baseline. Participants were prospectively followed for 4 years (Q1–Q3: 3.0–5.0) and stratified into baseline TMAO tertiles. Cox proportional hazards and linear and logistic mixed effect models were employed adjusting for risk factors.ResultsSubjects in the highest TMAO tertile were older (75.4±8.1 vs 70.6±8.5 years, p2(35.6–62.5) vs 67.3 mL/min/1.73 m2(57.8–78.9), pConclusionsHigh levels of TMAO are associated with increased risk of cardiovascular mortality and cerebral infarction in patients with atrial fibrillation.Trial registration numberNCT02105844.