The main purpose of this study is to investigate if treatment of obstructive sleep apnea syndrome (OSAS) by continuous positive airway pressure (CPAP) given by a traditional CPAP device administering a fixed air pressure, or by one automatic CPAP device ("Somnosmart2", Weinmann, Hamburg) administering variable pressures, have different effects on sympathetic nervous system tone (as reflected by urinary excretion of norepinephrine and its catabolite normetanephrine) and on blood pressure.

A nocturnal home cardiorespiratory recording will performed for diagnosis of OSA. During the same night, patients will be asked to collect their urines. Twentyfour-hour ambulatory BP monitoring (ABPM) will be performed starting from the next morning. After analysis of the polygraphic recording, subjects with AHI <15 will be excluded from the study, while CPAP will be proposed as a treatment to the other patients.

Before starting treatment, a full night standard polysomnography will be performed: in subjects assigned to fixed CPAP for attended CPAP titration; in the other subjects during application of the Somnosmart2 device, to verify if obstructive events are adequately eliminated by it.

After analysis of polysomnography, patients will be given a fixed CPAP or the auto-CPAP device for nocturnal home use. Two months later, compliance to treatment will be verified measured by the in-built time counter of the devices. Then, cardiorespiratory monitoring during application of the device used for treatment, nocturnal urine collection and 24-hour ABPM will be repeated.

Inclusion Criteria:

• Obstructive sleep apnea syndrome deserving CPAP treatment

Exclusion Criteria:

• current pharmacological treatment