BACKGROUND: Ultrasonography has been used for the diagnosis of many kinds of lung conditions, but few studies have investigated ultrasound for the diagnosis ofneonatal pulmonary atelectasis (NAP). In this study, we evaluated the usefulness of lung ultrasonography for the diagnosis of NPA. METHODS: From May 2012 to December 2013, 80 neonates with NPA and 50 neonates without lung disease were enrolled in this study. Each lung ofevery infant was divided into the anterior, lateral, and posterior regions by the anterior and posterior axillary lines. Each region was scanned carefully with the probe perpendicular or parallel to the ribs. The ultrasound findings were confirmed by chest radiograph (CXR) or CT scan. RESULTS: Sixty of the 80 patients with signs of NPA on lung ultrasound also had signs of NPA on CXR (termed focal-type atelectasis), and the other 20 patients had signs of NPA on chest CT scan while there were no abnormal findings on CXR (termed occult lung atelectasis). In patients with NPA, the main ultrasound findings were large areas of lung consolidation with clearly demarcated borders, air bronchograms, pleural line abnormalities, and absence of A-lines, as well as the presence of lung pulse and absence of lung sliding on real-time ultrasound. The sensitivity of lung ultrasonography for the diagnosis of NPA was 100%, whereas the sensitivity of CXR was 75%. Large areas of lung consolidation with clearly demarcated borders were only observed in patients with NPA. CONCLUSIONS: Lung ultrasonography is an accurate and reliable method for diagnosing NPA; most importantly, it can find those occult lung atelectasis that could not be detected on CXR. Routine lung ultrasonography is a useful method of diagnosing or excluding NPA in neonates. © 2015 American College of Chest Physicians.