The protocol implies the gathering of basic data (appendix 3), that will be further obtained when the patients meets the criterion of Pa02/Fi02 less than 300. Gathering of data ends 28 days after day 1 (IT MAY NOT CORRESPOND TO DAY 28 AFTER ADMISSION TO ICU); afterwards, data such as date of extubation, of discharge from ICU and from hospital, and condition upon discharge from hospital will be gathered.