Abstract Objective Critically ill patients admitted in ICU because of COVID-19 infection display severe hypoxemic respiratory failure. The Surviving Sepsis Campaign recommends oxygenation through high-flow nasal cannula over non-invasive ventilation. The primary outcome of our study was to evaluate the effect of the addition of a surgical mask on a high-flow nasal cannula system on oxygenation parameters in hypoxemic COVID-19 patients admitted in ICU who do not require urgent intubation. The secondary outcomes were relevant changes in PaCO2 associated with clinical modifications and patient’s feelings. Design We prospectively assessed 21 patients admitted in our mixed Intensive Care Unit of the Cliniques Universitaires Saint Luc. Main results While FiO2 was unchanged, we demonstrate a significant increase of PaO2 (from 59 (± 6), to 79 mmHg (± 16), p