Purpose To analyse the ultrasonographic findings of community-acquired pneumonia (CAP) and its efficacy for diagnosis of CAP compared with chest X-ray (CXR). Methods Patients who presented to the Emergency Department with suspected CAP were included in the study. Bedside ultrasonography was performed at each intercostal space in the midclavicular, anterior axillary, midaxillary and paravertebral lines. Any pulmonary consolidation, focal interstitial pattern, pleural-line abnormalities and subpleural lesions were recorded, and the numbers of subpleural lesions and intercostal spaces with pleural-line abnormalities were counted. All patients received bedside CXR and CT. Using CT scan as the gold standard, ultrasonography findings were compared between CAP group and non-CAP group, and between CAP patients with CT showing consolidation or diffuse ground-glass opacification. The sensitivity of ultrasonography was compared with CXR for the diagnosis of CAP. Results Of 179 patients included in the study, 112 were diagnosed with CAP by CT. Patients in CAP group were more likely to have consolidation (p<0.001), focal interstitial pattern (p<0.001) and had higher number of subpleural lesions (p<0.001) and intercostal spaces with pleural-line abnormalities (p<0.001) on ultrasound than those without CAP. CAP patients whose CT showed consolidation were more likely to have consolidation (p<0.001) and had lower numbers of subpleural lesions (p<0.001) and intercostal spaces with pleural-line abnormalities (p<0.001) compared to CAP patients whose CT showed diffuse ground-glass opacification. The diagnostic sensitivity, specificity, and accuracy for ultrasonography and CXR were 94.6% versus 77.7% (p<0.001), 98.5% versus 94.0% (p=0.940) and 96.1% versus 83.8% (p<0.001), respectively. Conclusions Lung ultrasonography has a better diagnostic sensitivity and accuracy for diagnosing CAP compared with CXR.