Aim: To evaluate causes of noninvasive mechanical ventilation (NIMV) failure. The rate of NIMV failure in respiratory failure due to chronic obstructive pulmonary disease (COPD) exacerbations was reported as 5%-40%. Materials and methods: The necessity of endotracheal intubation was accepted as NIMV failure. The causes of NIMV failure were assessed in 54 patients (45 males; mean age: 67.7 ± 11.0 years) treated with NIMV because of COPD exacerbations and respiratory failure in an intensive care unit (ICU). Results: There was NIMV failure in 20 patients (37.0%). The rates of hospital-acquired pneumonia and in-hospital mortality were higher (P = 0.003 and P = 0.002, respectively) and the duration of ICU stay was longer (P < 0.0001) in patients with NIMV failure. On admission, arterial pH, serum albumin, and Glasgow Coma Scale levels were lower (P = 0.032, P = 0.024, and P = 0.013, respectively) in the NIMV failure group. Arterial pH was lower (P = 0.039) and respiratory rate was higher (P = 0.010) after 1 h, and the PaO2/FiO2 rate was lower (P = 0.017) and respiratory and heart rates were higher (P = 0.002 and P = 0.020, respectively) after 3 h in the NIMV failure group. Conclusion: The present data strongly suggest that baseline and follow-up clinical and arterial blood gas evaluations can give important clues about NIMV failure in COPD exacerbations. © TÜBITAK.