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Introduction: The accuracy of echocardiography in evaluating left ventricular (LV) diastolic function has not been validated in children. The objective of this study was to compare echocardiographic and gold-standard measures of LV diastolic function in children. Methods: Patients undergoing routine left heart catheterization were prospectively enrolled. Pressure-volume loops (PVL) were obtained via conductance catheters. The end-diastolic pressure-volume relationship was obtained via balloon occlusion of the vena cavae. PVL measures of diastolic function were divided into early active relaxation (the isovolumic relaxation time constant, tau), and ventricular stiffness (the chamber stiffness constant, β). End-diastolic pressure (EDP) was also recorded. Echocardiographic measures of diastolic function were derived from spectral Doppler, tissue Doppler, and 2D speckle-tracking. The relationships between PVL and echocardiographic measures were determined using Spearman’s correlation. Results: Of 24 patients, 18 patients were s/p heart transplant, 5 patients had a small patent ductus arteriosus or coronary fistula. Mean age was 9.1 ± 5.6 years. The median τ was 24.9 ms (IQR 22.8 - 28.4 ms), median β was 0.094 (IQR 0.035 - 0.154), and median EDP was 9 mmHg (IQR 8 - 13 mmHg). Statistically significant correlations between invasive and echocardiographic measures of diastolic function are reported in the Table. No echocardiographic measures correlated with β. Conclusion: Early diastolic echocardiographic measures correlate with tau and may accurately represent early active relaxation in children. Modest associations exist between echocardiographic measures and EDP. The use of these non-invasive measures in accurately assessing LV diastolic function appears promising in children. However, no echocardiographic measures correlate with chamber stiffness. The development of such measures merits further study.