Background:The diagnosis of pneumonia in children is challenging, given the wide overlap of many of the symptoms and physical examination findings with other common respiratory illnesses. We sought to derive and validate the novel Pneumonia Risk Score (PRS), a clinical tool utilizing signs and symptoms available to clinicians to determine a child's risk of radiographic pneumonia. Methods: We prospectively enrolled children 3 months to 18 years in whom a chest radiograph (CXR) was obtained in the emergency department to evaluate for pneumonia. Before CXR, we collected information regarding symptoms, physical examination findings, and the physician-estimated probability of radiographic pneumonia. Logistic regression was used to predict the presence of radiographic pneumonia, and the PRS was validated in a distinct cohort of children with suspected pneumonia. Results: Among 1181 children included in the study, 206 (17%) had radiographic pneumonia. The PRS included age in years, triage oxygen saturation, presence of fever, presence of rales, and presence of wheeze. The area under the curve (AUC) of the PRS was 0.71 (95% confidence interval [CI]: 0.68-0.75), while the AUC of clinician judgment was 0.61 (95% CI: 0.56-0.66) (P < 0.001). Among 2132 children included in the validation cohort, the PRS demonstrated an AUC of 0.69 (95% CI: 0.65-0.73). Conclusions: In children with suspected pneumonia, the PRS is superior to clinician judgment in predicting the presence of radiographic pneumonia. Use of the PRS may help efforts to support the judicious use of antibiotics and chest radiography among children with suspected pneumonia.