Background: Trying to differentiate serious bacterial infection (SBI) from a self-limiting illness in febrile infants seen in the pediatric emergency department (PED) is a significant challenge. The aim of the study was to determine the prevalence of dehydration and its relationship to SBI in febrile full-term newborns under 1 week of age seen in a PED. Methods: A retrospective observational study was carried out on all children younger than 8 days of age with fever who presented to a single, tertiary care, PED from January 2009 to April 2014. Dehydration was defined as plasma sodium >150 mmol/L or >10% loss of birth weight. SBI was defined by the presence of a positive culture in the blood, urine, cerebrospinal fluid; osteoarticular infection; bacterial enteritis; or pneumonia. The primary analysis was the proportion of children with dehydration. A secondary analysis compared proportion of infection according to hydration status. Results: Of the 895 children under 8 days of age who visited the PED, 69 consulted for fever. Seven patients were excluded because they were transferred from another hospital. Sixty-two eligible patients were included in the final analysis. Of these, 17 (27%) were dehydrated according to our definition. Only 2 patients had an SBI while 2 others had a final diagnosis of viral myocarditis and encephalitis, respectively. None of the 4 children with serious infection fulfilled our definition of dehydration, and all had a plasma sodium level lower than 145 mmol/L. Conclusions: Dehydration is frequently associated with fever in infants younger than 8 days of age seen in a PED. Early identification of dehydration may be useful in limiting the aggressive intervention in some of these infants.