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Background: Left atrial volume (LAV) is an important predictor of adverse cardiovascular events. LAV measurements are routinely performed using 2D echocardiography (2DE). This methodology is limited because it is view-dependent and relies on geometrical assumptions regarding LA shape. Real-time 3D echocardiography (RT3DE) is an attractive alternative for the evaluation of LAV. However, few studies have validated RT3DE-derived LAV measurements against an accepted independent standard, because of the lack of appropriate analysis tools. We studied in a multicenter setting the accuracy and reproducibility of RT3DE LAV measurements using new dedicated volumetric software, side-by-side with 2DE, using cardiac magnetic resonance (CMR) imaging as a reference.Methods: We studied 42 patients with a wide range of LAV who underwent CMR (1.5T), 2DE and full-volume RT3DE imaging (Philips, iE33) on the same day. All images were analyzed to obtain maximal LAV: CMR images using standard commercial tools, 2DE - biplane area-length (AL) technique, and 3DE - TomTec LA Function software, while excluding the LA appendage. Inter-technique comparisons included linear regression and Bland-Altman analyses. Reproducibility of all 3 techniques was assessed by calculating coefficients of variation in blinded repeated measurements.Results: RT3DE-derived LAV values showed higher correlation with CMR than 2DE measurements (r=0.91 vs. r=0.85). While 2DE underestimated LAV by 28±42ml, 3DE resulted in a minimal positive bias of 5±35ml. Inter- and intra-observer variability of 2DE and 3DE measurements were similar (7-10%) and roughly twice higher than CMR (4-5%).Conclusion: When compared to CMR reference, 3DE-derived LAV measurements are more accurate than the conventional 2DE-based AL analysis with similar reproducibility.