BackgroundIdiopathic dilated cardiomyopathy prognosis is influenced by endothelial dysfunction in adults with heart failure but this information is lacking in children. ObjectiveEvaluate endothelial function in children with IDCM and HF. MethodThe idiopathic dilated cardiomyopathy group (IDCM) was composed of 15 children with left ventricular ejection fraction <40%. The heart failure group (HF) was composed of 16 children with left ventricular ejection fraction (LVEF) >50% who had previously been diagnosed with idiopathic dilated cardiomyopathy. The control group (CON) was composed of 11 healthy children. The endothelial function in all children was assessed via reactive hyperemia index (RHI) – peripheral artery tonometry testing and cardiopulmonary exercise test. ResultsIDCM were significantly shorter than CON (1.2×1.4 in m, respectively) and they received higher carvedilol, captopril, and furosemide dosage. CON presented RHI=1.53±0.26. HF presented RHI=1.14±0.31. IDCM presented RHI=1.04±0.24. CON showed a RHI 31% higher than HF and 25% higher than IDCM, significantly. LVEF was moderately correlated to the RHI (r=0.46). For each unit of RHI, there was an addition of 10.57 in maximal oxygen consumption (VO2 max). Lower RHI values correlated with lower VO2 max values. ConclusionChildren with IDCM and HF had endothelial dysfunction. The endothelium remained dysfunctional even after improvement in cardiac function.