Objective: The purpose of this study is to describe the prevalence of an infiltrate on chest radiograph in children between the ages of three and 36 months presenting to the Emergency Department with fever of 103F (39.4C) or greater. Methods: This prospective study was conducted at an academic ED that treats 16,000 pediatric patients per year. Consecutive enrollment of subjects meeting entry criteria was performed by the principal investigator, during all days of the week, when the principal investigator was present. Patients were eligible for enrollment if they were aged 90 days to 36 months, had a rectal temperature of 103F or greater, were immunocompetent, had not received antibiotic therapy prior to evaluation, and did not have otitis media or pneumonia on physical examination. Patients were evaluated by physical examination, rectal temperature, complete blood count, blood culture, catheter-obtained urine analysis and culture, pulse oximetry, and chest radiography. All chest radiographs were interpreted by the attending emergency physician and by an attending pediatric radiologist.Results: Over a 14 month period, 93 patients were enrolled with a mean age of 13.9 months (SD = 8.4, range = 3 to 36 months). The mean temperature was 104.1F (range 103F to 106.2F). Two chest radiographs were read as positive for infiltrate by the pediatric radiologist (2.15%, 95% C I= 0.3 to 7.6). The emergency medicine attending read three radiographs as positive for infiltrate (3.23%, 95% CI= 0.7 to 9.1). The emergency medicine attending considered one radiograph positive that was read as negative by the pediatric radiologist. Conclusions: The prevalence of abnormal chest radiographs in patients from 90 days to 36 months of age with fever of 103F or greater and no pneumonia on physical exam is low, approximately 2% to 3%. Further study is underway to evaluate signs and symptoms which may predict which patients will have an abnormal chest radiograph.