OBJECTIVES: To evaluate the proportion of children presenting to the emergency department (ED) with altered mental status who demonstrate nonconvulsive seizures on reduced-lead electroencephalography (EEG), and to further investigate the characteristics, treatment, and outcomes in these patients compared with patients without nonconvulsive seizures. STUDY DESIGN: In this retrospective cohort study, we reviewed the database and medical records of pediatric patients (aged <18 years) in a single ED between May 1, 2016, and April 30, 2018. We first determined the proportion of nonconvulsive seizures among patients with altered mental status (Glasgow Coma Scale <15). We then compared the clinical presentation, demographic data, clinical diagnosis, EEG results, treatment, and outcomes of patients with altered mental status with nonconvulsive seizures and those without nonconvulsive seizures. RESULTS: In total, 16.9% of the patients with altered mental status (41 of 242; 95% CI, 12.2%-21.6%) evaluated by EEG had detectable nonconvulsive seizure, equivalent to 4.4% (41 of 932) of all patients with altered mental status presenting at our hospital. More than 80% of patients monitored for nonconvulsive seizures had a previous history of seizures, often febrile. Patients with nonconvulsive seizures were older (median, 68.5 vs 36.1 months) and had a higher Pediatric Cerebral Performance Category score at presentation (median, 2.0 vs 1.0). In addition, the proportion of patients admitted to the intensive care unit was significantly higher in the patients with nonconvulsive seizures (30.3% vs 15.0%). However, total duration of hospitalization, neurologic sequelae, and 30-day mortality rate did not differ between the 2 groups. CONCLUSIONS: A relatively high percentage of pediatric patients with altered mental status in the ED experience nonconvulsive seizures. The use of reduced-lead EEG monitoring in the ED might facilitate the recognition and treatment of nonconvulsive seizures, especially among patients with a history of seizures.