Obstructive sleep apnea syndrome (OSAS) affects 2% and 4% of middle-aged women and men respectively, and is associated with an increased risk of cardiovascular complications. Therefore, cardiovascular consequences must be evaluated in the clinical management of OSAS and the study of cardiac functional parameters could be particularly useful. Reported findings are conflicting with respect to the influence of OSAS and nasal continuous positive airway pressure (nCPAP) on left ventricular function. In this study, we assess cardiac dysfunction with echocardiographic parameters, such as LVEF, left ventricular mass, ratio of E to A (E/A) and mitral deceleration time (DT) from the mitral inflow velocity, and Tei-index, or plasma brain natriuretic peptide (BNP) level at baseline, and 1 and 3 months after nCPAP treatment. The main endpoint is the comparison of echocardiographic parameters and BNP between OSAS patients and control subjects, and the changes of them after short-term of nCPAP treatment.