Background: We prospectively evaluated the applicability and effect of noninvasive ventilation (NIV) in acute respiratory failure and tried to find out the parameters that could predict successful application of NIV. Methods: Twenty-six out of 106 patients with either acute ventilatory failure (VF: PaCO2> 43 mm Hg with pH <7.35) or oxygenation failure (OF: PaO2/FIO2 < 300 mm Hg with pH> or = 7.35) requiring mechanical ventilation were managed by NIV (CPAP + pressure support, or BiPAP) with face mask. Eleven out of 19 cases with VF (57.9%) (M: F=7: 4, 55.4 +/-14.6 yrs) and 15 out of 87 cases with OF (17.2%) (M: F= 12: 3, 50.6+/-15.6 yrs) were suitable for NIV. Respiratory rates, arterial blood gases and success rate of NIV were analyzed in each group. Results: 81.8% (9/11) of VF and 40% (6/15) of OF were successfully managed on NIV and were weaned from mechanical ventilator without resorting to endotracheal intubation. Complications were noted in 2 cases (nasal skin necrosis 1, gaseous gastric distension 1). In NIV for ventilatory failure, the respiration rate was significantly decreased at 12 hour of NIV (34+/-9 /min pre-NIV, 26+/-6 /min at 12 hour of NIV, p=0.045), while PaCO2 (87.3+/-20.6 mm Hg pre-NIV, 81.2+/-9.1 mm Hg at 24 hour of NIV) and pH (7.26 +/-0.04, 7.32 +/-0.02, respectively, p< 0.05) were both significantly decreased at 24 hour of NIV. In NIV for oxygenation failure, PaO2 were not different between the successful and the failed cases at pre-NIV and till 12 hours after NTV. The PaO2/FIO2 ratio, however, significantly improved at 0.5 hour of NIV in successful cases and were maintained at around 200 mm Hg (n=6: at baseline, 0.5h, 6h, 12h: 120.0+/-19.6, 218.9+/-98.3, 191.3+/-55.2, 232.8+/-17.6 mm Hg, respectively, p=0.0211), but it did not rise in the failed cases (n=9: 127.9+/-63.0, 116.8+/-24.4, 100.6 +/-34.6, 129.8+/-50.3 mm Hg, respectively, p=0.5319). Conclusion: From the above results we conclude that NIV is effective for hypercapnic respiratory failure and its success was heralded by reduction of respiration rate before the reduction in PaCO2 level. In hypoxic respiratory failure, NIV is much less effective, and the immediate improvement of PaO2/FIO2 ratio at 0.5h after application is thought to be a predictor of successful NIV.