BACKGROUND AND OBJECTIVESTo determine the visibility of pleural lung sliding in alveolar–interstitial syndrome (AIS) in patients on mechanical ventilation at two different time points, as a confirmatory ultrasonographic method for excluding pneumothorax. METHODSFifty-two mechanically ventilated patients in the semirecumbent position in a surgical/neurosurgical intensive care unit with ultrasonographic lung ‘comet tails’ in three upper anterolateral intercostal spaces, indicating the presence of AIS, were scanned for lung sliding in the same three intercostal spaces with a linear 5–10 MHz transducer after starting mechanical ventilation and on weaning trials. Pneumothorax and atelectasis were excluded by chest radiograph. RESULTSAbsent lung sliding was found in 22.7% of intercostal spaces scanned after starting mechanical ventilation and in 21.2% of scans taken on weaning trials. The lowest invisible rate was in patients with acute heart failure and the highest in patients with acute respiratory distress syndrome. CONCLUSIONLung sliding specificity in AIS during mechanical ventilation was 78%. Our opinion is that different levels of airway pressure between starting mechanical ventilation and weaning trials have no influence on lung sliding visibility.