The patients will be recruited from the Sleep Laboratory, Heart Institute (InCor), University of São Paulo Medical School. Male adults with a sleep study within 1 month showing severe OSA (>30 events of apnea and hypopnea per hour of sleep) and naive to treatment will be considered for the study. Age and body mass index matched controls will be recruited from the hospital staff and their relatives. All volunteers will be previously screening for the risk of OSA with Berlin Questionnaire. To minimize confounding risk factors, we will exclude subjects older than 60 years and those with a body mass index (BMI) >40 kg/m2, diabetes mellitus, hypertension, cerebrovascular disease, valvular heart disease, renal failure, current or past smoking history, and chronic use of any medication. Hypertension will be excluded after 3 or more normal blood pressure values (<140/90mm Hg) obtained on separate occasions with a conventional mercury sphygmomanometer.