Fragmented QRS (fQRS) is an easily evaluated noninvasive parameter in electrocardiograms (ECGs) for predicting cardiac adverse events. Our study aimed to evaluate whether fQRS could be used as a risk factor to predict a major adverse cardiac event (MACE) in children with idiopathic dilated cardiomyopathy (DCM). Our retrospective study enrolled 63 patients aged ≤ 19 years who were newly diagnosed with idiopathic DCM at Samsung Medical Center from 2003 to 2014, and followed up until December 2018. Demographic data, ECGs, and echocardiography were reviewed and analyzed for their possible links with adverse outcomes. The median age was 14.0 months and the median length of follow-up was 65.0 months. Age, QRS duration, and ejection fraction in M-mode at diagnosis were significantly different between the fQRS and non-fQRS groups (P = 0.026; P = 0.001; and P = 0.020). Significant difference between the fQRS and non-fQRS groups was found in patients with a MACE (P = 0.016, odd ratio 3.643) or any arrhythmias (P = 0.008, odd ratio 6.563). The MACE-free survival rate showed a significant difference in terms of fQRS (P = 0.003; P = 0.007; and P = 0.027). In univariate analyses, age, QRS duration, corrected QT, positive fQRS, and the number of leads with fQRS were significant predictors of MACEs. Among the above factors, positive fQRS at diagnosis was a strongly significant predictor of adverse outcomes in multivariate analyses (hazards ratio 94.529, P = 0.001). Fragmented QRS complex at diagnosis could be used as a strong predictor for cardiac adverse outcomes in pediatric patients with idiopathic DCM.