Purpose: Electroencephalography (EEG) is an effective test in predicting severe cortical dysfunction associated with poor outcomes in adult patients, but its value in pediatric patients remains incomplete. Here, we assessed the prognostic value of EEG regarding sedative history and various etiologies in pediatric patients who had undergone EEGs at the pediatric intensive care unit of Severance Hospital for 5 years. Methods: We performed a retrospective study of total 113 pediatric patients who met our criteria. In-hospital mortality was measured for the primary outcome. Results: In-hospital mortality was observed in 43 patients (38.1%) and sedatives were used in 37 patients (32.7%). Patients who showed in-hospital mortality were more like to have higher EEG background scores and absent EEG reactivity (P<0.001 for both). The prognostic values of these EEG factors were statistically significant in non-sedated patients (P<0.001 for both) whereas they were not significant in sedated patients (P=0.980 and P=0.336, respectively). In a multivariable regression analysis conducted in non-sedated patients, higher EEG background score and absence of EEG reactivity were independently associated with higher mortality rate (P=0.015 and P=0.001, respectively). They also showed high prognostic values of mortality in non-sedated patients, irrespective of each etiology (hypoxic ischemic encephalopathy [HIE]: P=0.013 and P=0.021, respectively; non-HIE structural brain disease: P=0.001 and P=0.002, respectively; non-structural brain dysfunction: P<0.001 for both). Conclusion: Our findings prove that both an abnormal background rhythm and the absence of reactivity in early EEG can be independent factors associated with mortality in non-sedated critically ill children irrespective of etiology.