The purpose of the study is to investigate the effect of acetazolamide as a treatment for sleep related breathing disturbances in patients with the obstructive sleep apnea syndrome living at low altitude during a sojourn at moderate altitude

Inclusion Criteria:

• Obstructive sleep apnea syndrome, successfully on CPAP therapy
• Residence at low altitude (<800m)
• Obstructive apnea/hypopnea index >20/h and a complaint of excessive daytime sleepiness before introduction of CPAP therapy.
• >15 oxygen desaturations/h (>3% dips) during an ambulatory nocturnal pulse oximetry performed at the end of a 4-night period without CPAP

Exclusion Criteria:

• Sleep disorders other than OSA.
• More than mild cardiovascular disease, unstable or recently diagnosed (within the last 6 months) cardiovascular disease such as arterial hypertension, coronary artery or cerebrovascular disease.
• Any lung disease, pulmonary hypertension.
• Chronic rhinitis.
• Treatment with drugs that affect respiratory center drive (benzodiazepines or other sedatives or sleep inducing drugs, morphine or codeine derivates), stimulants (modafinil, methylphenidate, theophylline)
• Internal, neurologic or psychiatric disease that interfere with sleep quality.
• Previous intolerance to moderate or low altitude (<2600m).
• Exposure to altitudes >1500m for >1 day within the last 4 weeks before the study