Background <p>This study aimed to compare serum and cerebrospinal fluid (CSF) copeptin levels among children with febrile seizures, febrile illness without seizures, and meningitis; to investigate the relationship between serum and CSF copeptin levels; and to assess the potential diagnostic utility of these biomarkers.</p> Methods <p>This study included children aged 1 month to 18 years who presented to the pediatric emergency department with fever or seizures and underwent lumbar puncture because of signs of meningeal irritation or the absence of an identifiable source of fever. The patients were classified into three groups: the Meningitis Group, the Febrile Illness Without Seizures Group, and the Febrile Seizure Group. In addition, a control group consisting of healthy children was included. Serum and CSF copeptin levels were measured in the patient groups, whereas only serum copeptin levels were measured in the control group.</p> Results <p>Serum copeptin levels were significantly higher in the Febrile Seizure Group than in both the Febrile Illness Without Seizures Group and the Control Group. Univariate linear regression analysis demonstrated a weak but statistically significant positive association between serum and CSF copeptin levels (<i>p</i> = 0.040), indicating that CSF copeptin levels could be estimated from serum copeptin levels using the following formula: CSF copeptin level = 2.02 + 0.05 × serum copeptin level. In multivariable linear regression analysis, C-reactive protein (CRP) and CSF glucose levels were identified as independent predictors of CSF copeptin levels, irrespective of age and sex (<i>p</i> = 0.009 and <i>p</i> = 0.004, respectively). CSF copeptin measurement was not superior to serum copeptin measurement for distinguishing children with febrile seizures, or children with febrile seizures and/or meningitis, from those with febrile illness without seizures.</p> Conclusion <p>Serum copeptin levels were higher in children with febrile seizures than in those with febrile illness without seizures and in healthy controls. However, because of the very small meningitis sample size, the meningitis-related findings should be considered exploratory and interpreted with caution. Larger prospective studies are required to determine whether copeptin has clinically meaningful utility in distinguishing febrile seizures from meningitis.</p>