<p>Red cell distribution width is a prognostic marker of adverse outcomes in adults with heart disease, including congenital heart disease. However, its significance has not been clarified in pediatric patients. We investigated the relationship between red cell distribution width and cardiovascular events in school-age children with congenital heart disease and uniform reference intervals. We performed a retrospective cohort study of pediatric patients with congenital heart disease between January 2018 and March 2023. Children aged 7–15 years were included, and the results of the first blood test during the study period were considered as baseline data. Cardiovascular events were defined as all-cause mortality, non-scheduled hospitalizations for cardiovascular reasons, heart failure, arrhythmias, thromboembolism, and unplanned intervention. In total, 101 children were included. The median red cell distribution width was 12.3% (interquartile range 11.9–12.8%). Cardiovascular events occurred in 15 patients (14.9%) during follow-up. The Kaplan–Meier analysis showed significantly worse cardiovascular event-free survival in the higher red cell distribution width group. Univariate Cox regression revealed that a higher red cell distribution width group (<i>p</i> &lt; 0.001), Fontan circulation (<i>p</i> &lt; 0.001), oxygen saturation (<i>p</i> &lt; 0.001), aspartate aminotransferase (<i>p</i> &lt; 0.001), alanine aminotransferase (<i>p</i> &lt; 0.001), and gamma-glutamyl transpeptidase (<i>p</i> &lt; 0.001) were associated with cardiovascular events. When adjusted for other variables respectively, only a higher red cell distribution width group and Fontan circulation remained independently significant. The red cell distribution width and Fontan circulation were independently associated with cardiovascular events in school-age children with congenital heart disease.</p>