Obstructive sleep apnea syndrome (OSAS) increases the risk of cardiovascular events. Sympathetic nervous system and hypothalamic-pituitary-adrenal (HPA) axis activation may be the mechanism of this relationship. The aim of this study was to evaluate HPA axis and ambulatory blood pressure monitoring (ABPM) in obese men with and without OSAS and to determine whether nasal continuous positive airway pressure therapy (nCPAP) influenced responses. Twenty-four-hour ambulatory blood pressure monitoring and overnight cortisol suppression test with 0.25 mg of dexamethasone were performed in 16 obese men with OSAS and 13 obese men controls. Nine men with severe apnea were reevaluated three months after nCPAP therapy. Body mass index and blood pressure of OSAS patients and obese controls were similar. In OSAS patients, the percentage of fall in systolic blood pressure at night (p=0.027) and salivary cortisol suppression post DEX (p=0.038) were lower, while heart rate (p=0.022) was higher compared with obese controls. After nCPAP therapy, patients showed a reduction in heart rate (p=0.036) and a greater cortisol suppression after dexamethasone (p=0.001). No difference in arterial blood pressure (p=0.183) was observed after 3 months of nCPAP therapy. Improvement in cortisol suppression was positively correlated with an improvement in apnea-hypopnea index during nCPAP therapy (r= 0.799; p=0.010). In conclusion, men with OSAS present increased post dexamethasone cortisol levels and heart rate which were recovered by nCPAP.