The efficacy of noninvasive ventilation (NIV) to avoid intubation and improve survival was assessed in 105 patients with severe acute hypoxemic respiratory failure (arterial O2 tension or saturation persistently 60 mm Hg or less or 90% or less, respectively; breathing conventional Venturi oxygen at a maximal concentration [50%]), excluding hypercapnia, admitted into intensive care units of three hospitals. Patients were randomly allocated within 24 hours of fulfilling inclusion criteria to receive NIV (n=51) or high-concentration oxygen therapy (n=54). The primary end-point variable was the decrease in the intubation rate. Both groups had similar characteristics. Compared with oxygen therapy, NIV decreased the need for intubation (13, 25% vs. 28, 52%, p=0.010), the incidence of septic shock (6, 12% vs. 17, 31%, p=0.028), and the intensive care unit mortality (9, 18% vs. 21, 39%, p=0.028) and increased the cumulative 90-day survival (p=0.025). The improvement of arterial hypoxemia and tachypnea was higher in the noninvasive ventilation group with time (p=0.029 each). Multivariate analyses showed NIV to be independently associated with decreased risks of intubation (odds ratio, 0.20; p=0.003) and 90-day mortality (odds ratio, 0.39; p=0.017). The use of noninvasive ventilation prevented intubation, reduced the incidence of septic shock, and improved survival in these patients compared with high-concentration oxygen therapy.