Objective: (i) Analyze the effect of altitude above the sea level on the mortality rate in patients undergoing invasive mechanical ventilation. (ii) Validate the traditional equation for adjusting PaO2/FiO2 according to the altitude. Design: A prospective, observational, multicenter and international study conducted during August 2016. Patients: Inclusion criteria: (i) age between 18 and 90 years old, (ii) admitted to intensive care unit (ICU) situated at the same altitude above the sea level (AASL) in which the patients has stayed, at least, during the previous 40 days and (iii) received invasive MV for at least 12 h. Material and methods: All variables were registered the day of intubation (day 0). Patients were followed until death, ICU discharge or day 28. PaO2/FiO2 ratio was adjusted by the AASL according to: PaO2/FiO2 * (barometric pressure/760). Categorical variables were compared...