Rational: Community-acquired pneumonia (CAP) is a frequent infectious disorder in patients visiting the ED. CAP is responsible for high morbidity and associated-mortality is increasing in Western countries. CAP corresponds to invasion of the lung by pathogens. Diagnosis depends on clinical and X-ray assessment. However, these signs and symptoms are poorly specific and are often lacking. As prognosis depends on precocious and fitted antimicrobial treatment, making CAP diagnosis in a short time span (4-8 hours) is mandatory. Preliminary studies suggest that chest CT-scan could over-performed X-ray for diagnosis of CAP. Consensus conferences suggest the use of CT-scan in patients with uncertain diagnosis and unusual presentation and outcome. Because CT-scan is currently easily available, its use in a first intent is questionable for ED patients with suspected CAP.