Inclusion Criteria:  Men and women, age 35-75 y, living at low altitude (<800 m).  COPD diagnosed according to the Global Initiative for Obstructive Lung Disease (GOLD) guidelines, FEV1 40-80% predicted, pulse oximetry  ≥92%, PaCO2 <6 kPa, breathing ambient air at 760 m.  One of the following early signs and/or symptoms of impending altitudeillness identified by self-monitoring during ascent to or stay at 3100 m: Pulse oximetry SpO2≤84% Headache or nausea/vomiting or fatigue/weakness or dizziness/lightheadedness of at least moderate intensity  Exclusion Criteria:  COPD exacerbation, very severe COPD with hypoxemia or hypercapnia at 760 m (see above). Other lung disease, relevant comorbidities (such as uncontrolled cardiovascular disease, i.e., unstable arterial hypertension, coronary artery disease; previous stroke; obesity (body mass index >35 kg/m2); internal, neurologic, rheumatologic or psychiatric disease; current heavy smoking (>20 cigarettes per day). Renal failure and/or allergy to sulfonamides. Patients who do not have early signs and/or signs of impending altitudeillness by self-monitoring (as defined above) at 3'100m will not be included.