Background <p>Cardiac magnetic resonance (CMR) enables the quantitative characterization of myocardial tissue through parametric myocardial tissue mapping. It shows a strong correlation with invasive histologic techniques in assessing diffuse myocardial fibrosis using native T1 mapping and extracellular volume (ECV), as well as in assessing myocardial edema with T2 mapping. Pediatric applicability is currently limited by a lack of specific reference values for this population, with no recommendations available from the Society of Cardiac Magnetic Resonance.</p> Objective <p>To determine reference values for global left ventricular native T1, T2 mapping, and extracellular volume fraction and to assess the influence of age, heart rate, body surface area, and sex on these parameters in children and adolescents.</p> Materials and methods <p>Data were retrospectively collected between 2018 and 2024. A total of 194 healthy children aged up to 17&#xa0;years underwent CMR. The participants had a mean age of 12.6 (3.4) CI [12.1; 13.1] years (range 0.3–17), and 41% were female. A 1.5-T Siemens MAGNETOM Avanto Fit scanner was used. Native T1 mapping was performed at the base, mid-chamber, and apex of the left ventricle in the short-axis view at end-diastole using a MOLLI sequence, before and 15&#xa0;min after the administration of a gadobutrol-based contrast agent. T2 mapping was performed before contrast administration at the base, mid-chamber, and apex of the left ventricle at end-diastole using a dark blood turbo spin echo sequence featuring a T2 preparation pulse and a balanced steady-state free precession (bSSFP). Linear regression models were employed to explore the relationships between native T1, ECV, and T2 mapping values and heart rate, age, and body surface area (BSA).</p> Results <p>The results showed a mean global native T1 of 1004±37&#xa0;ms, ECV of 25.9±3.6%, and T2 of 48.8±4&#xa0;ms. T1 and ECV values were negatively correlated with age and BSA (<i>P</i>&lt;0.001) and positively correlated with heart rate (<i>P</i>&lt;0.001). In contrast, T2 values were negatively correlated with age (<i>P</i>&lt;0.001) but not with heart rate (<i>P</i>=0.953 in the septum and <i>P</i>=0.445 in the free wall). Native T1 values were significantly higher in female patients than in males when analyzed separately for the interventricular septum and the left ventricular free wall (<i>P</i>=0.014 and <i>P</i>=0.001).</p> Conclusion <p>This study provides a median range of values in native T1, T2 mapping, and ECV in a pediatric population. The low number of participants under the age of 6 should be taken into account in the results of our study. Our findings corroborate that native T1 is influenced by both age and heart rate, while T2 is primarily affected by age alone. We observed higher native T1 values in females, highlighting the need for separate, sex-specific reference ranges. Therefore, age, heart rate, and sex should be considered in pediatric myocardial mapping.</p>