Prone position quickly imposed as a cornerstone in the management of patients with acute respiratory distress syndrome (ARDS) induced by coronavirus disease 2019 (COVID-19) [1, 2]. The response to prone position, in terms of oxygenation and respiratory mechanics, would differ according to the potential for lung recruitment in supine position in COVID-19-ARDS [3]. Recruitmentto-inflation ratio (R/I), which is measurable with almost all modern respirators, allows, at bedside, the distinction between patients with a low or high potential for lung recruitment [4], including in those with COVID19-ARDS [5]. This tool may henceforth help clinicians to set adequate positive end-expiratory pressure (PEEP) levels in ARDS [4]. Although not yet studied, the effects of other ventilatory strategies in ARDS should also depend on the R/I. Therefore, we conducted a prospective observational study to assess the effect of prone position in COVID-19-ARDS on respiratory mechanics and oxygenation according to the R/I ratio.