OBJECTIVE: Noninvasive ventilation (NIV) is an effective treatment in patients with acute exacerbation of COPD (AECOPD). However, it may induce post-hypercapnic metabolic alkalosis (MA). This study aims to evaluate the effect of acetazolamide (ACET) in AECOPD patients treated with NIV. PATIENTS AND METHODS: Eleven AECOPD patients, with hypercapnic respiratory failure and MA following NIV, were treated with ACET 500 mg for two consecutive days and compared to a matched control group. Patients and controls were non invasively ventilated in a bilevel posi tive airway pressure (BiPAP) mode to a standard maximal pressure target of 15-20 cmH2O. RESULTS: ACET intra-group analysis showed a significant improvement for PaCO2 (63.9 ± 9.8 vs. 54.9 ± 8.3 mmHg), HCO3- (43.5 ± 5.9 vs. 36.1 ± 5.4 mmol/L) and both arterial pH (7.46 ± 0.06 vs. 7.41 ± 0.06) and urinary pH (6.94 ± 0.77 vs. 5.80 ± 0.82), already at day 1. No significant changes in endpoints considered were observed in the con trol group at any time point. Inter group analysis showed significant differences between changes in PaCO2 and HCO3- (delta), both at day 1 and 2. Furthermore, the length of NIV treatment was significantly reduced in the ACET group compared to controls (6 ± 8 vs. 19 ±19 days). No adverse events were recorded in the ACET and control groups. CONCLUSIONS: ACET appears to be effective and safe in AECOPD patients with post NIV MA.