Background Epicardial adipose tissue (EAT) may induce left atrium (LA) wall inflammation and promote LA fibrosis. Therefore, simultaneous assessment of these two important atrial fibrillation (AF) risk factors would be desirable. Purpose To perform a comprehensive evaluation of 3D Dixon water-fat separated late gadolinium enhancement (LGE-Dixon) MRI by analysis of repeatability and systematic comparison with reference methods for assessment of fibrosis and fat. Study Type Prospective. Population Twenty-eight, 10, and 7 patients, respectively, with clinical indications for cardiac MRI. Field Strength/Sequence A 1.5-T scanner, inversion recovery multiecho spoiled gradient echo. Assessment Twenty-eight patients (age 58 +/- 19 years, 15 males) were scanned using LGE-Dixon. A 5-point Likert-type scale was used to grade the image quality. Another 10 patients (age 46 +/- 19 years, 9 males) were scanned using LGE-Dixon and 3D proton density Dixon (PD-Dixon). Finally, seven patients (age 62 +/- 14 years, 4 males) were scanned using LGE-Dixon and conventional LGE. The scan time, intraobserver and interobserver variability, and levels of agreement were assessed. Statistical Tests Students t-test, one-way ANOVA, and Mann-Whitney U-test were used; P < 0.05 was considered significant, intraclass correlation coefficient (ICC). Results The scan time (minutes:seconds) for LGE-Dixon (n = 28) was 5:01 +/- 1:40. ICC values for intraobserver and interobserver measurements of LA wall fibrosis percentage were 0.98 (95% CI, 0.97-0.99) and 0.97 (95% CI, 0.94-0.99) while of EAT were 0.92 (95% CI, 0.82-0.97) and 0.90 (95% CI, 0.80-0.95). The agreement for LA fibrosis percentage between the LGE-Dixon and the conventional LGE was 0.92 (95% CI, 0.66-0.99) and for EAT volume between the LGE-Dixon and the PD-Dixon was 0.93 (95% CI, 0.72-0.98). Conclusion LA fibrosis and EAT can be assessed simultaneously using LGE-Dixon. This method allows a high level of intraobserver and interobserver repeatability as well as agreement with reference methods and can be performed in a clinically feasible scan time. Evidence Level 2 Technical Efficacy Stage 3