The primary objective of this study is to compare the efficacy of treatment with HFNC (group A) compared to administration of oxygen therapy by Venturi mask (group B, standard therapy) in terms of reaching of endotracheal intubation criteria during acute respiratory failure due to severe pneumonia. Inclusion Criteria: Respiratory rate (RR) at rest ≥20 bpm or presence of respiratory distress (severe dyspnoea at rest or use of accessory respiratory muscles or abdominal paradox); PaO2 / FiO2 ≤250 during oxygenation with Venturi Oxygenation mask at FiO2 = 50% administered for at least 60 minutes; Diagnosis of pneumonia as the sole cause of acute respiratory failure. Randomization: 150 consecutive patients will be randomized either to High Flow Nasal Cannula Oxygenation (75 patients, HFNCO with flow ≥ 60 L/min and FiO2 to maintain SpO2 ≥ ) or Venturi Mask Oxygenation (control, 75 patients). Patients from both groups will be treated with antibiotic therapy according to the IDSA/ATS 2007 guidelines for community-acquired pneumonia and the IDSA/ATS 2016 guidelines for hospital-acquired pneumonia. Intubation Criteria: MAJOR CRITERIA: Cardiac or respiratory arrest Breathing pauses with loss of consciousness Severe hemodynamic instability Need for sedation MINOR CRITERIA (maintained for ≥1h): Reduction ≥30% of the value of the PaO2/FiO 2 compared to baseline Increased 20% if PaCO2 PaCO2 previous ≥40mmHg Worsening alertness as increased by one degree on the Kelly scale Persistence or onset of respiratory distress Vital parameters, Kelly scale and arterial blood gas analysis (BGA) will be performed on admission, and at 1, 24, at 48 hours, at the achievement of clinical stability, and whenever there is a clinical worsening. Patients enrolled in HFNC arm will continue HFNC oxygenation until clinical stability, defined as: Body temperature ≤ 37°C and ≥36°C for 24 consecutive hours Good ability in swallowing CRP and WBC normalization trend than the admission exams Hemodynamic stability Lack of respiratory distress SpO2 94-98% The primary outcome variable is the proportion of patients who reach the endotracheal intubation criteria - regardless of the actual intubation rate - within the first 48 hours of treatment. The primary analysis will be performed on the ITT population