Ultrasound results for 11 patients with HPN confirmed by CXR were compared with those for 100 healthy subjects. The observation of the hyperechogenic line of the pleuropulmonary surface (normal subjects) showed back-and-forth respiratory movements in every case which we call the "gliding sign." Ultrasonographic signs were shown in all patients with HPN. Visualizing the gassy effusion above the pleural fluid, the disappearance of the "gliding sign" (n = 11) indicates PN. The image of the HPN allows in addition a "curtain sign" which depicts the movement of air/fluid level (n = 11), the pulmonary collapse being calculated across the liquid window whose echostructure is analyzed. A "polymicrobullous" image (n = 2) caused by air microbubbles within the fluid effusion, is reported. We conclude that echography appears to be a new approach to diagnosing HPN, which is particularly useful during or after ultrasonically guided procedures including pleural drainage, and should be recognized by sonographers.