['High-frequency oscillatory ventilation (HFOV) has not been shown to be beneficial in the management of moderate-to-severe acute respiratory distress syndrome (ARDS). There is uncertainty about the actual pressure applied into the lung during HFOV. We therefore performed a study to compare the transpulmonary pressure (P L) during conventional mechanical ventilation (CMV) and different levels of mean airway pressure (mPaw) during HFOV.', 'This is a prospective randomized crossover study in a university teaching hospital. An esophageal balloon catheter was used to measure esophageal pressures (Pes) at end inspiration and end expiration and to calculate P L. Measurements were taken during ventilation with CMV (CMVpre) after which patients were switched to HFOV with three 1-h different levels of mPaw set at +5, +10 and +15\xa0cm\xa0H2O above the mean airway pressure measured during CMV. Patients were thereafter switched back to CMV (CMVpost).', 'Ten patients with moderate-to-severe ARDS were included. We demonstrated a linear increase in Pes and P L with the increase in mPaw during HFOV. Contrary to CMV, P L was always positive during HFOV whatever the level of mPaw applied but not associated with improvement in oxygenation. We found significant correlations between mPaw and Pes.'] Do effect of high-frequency oscillatory ventilation on esophageal and transpulmonary pressures in moderate-to-severe acute respiratory distress syndrome?