Background: There is limited information regarding myocardial alterations in patients with obstructive sleep apnea syndrome (OSAS) in the absence of pulmonary and cardiac comorbidity. In this study, we aimed to evaluate potential myocardial alterations of these patients and investigate the possible effects of OSAS-related pathological variations on left and right ventricular functions. Methods: We studied 107 consecutive patients who were referred to our sleep laboratory for clinically suspected OSAS and 30 controls without any history or symptoms of sleep-related disorders. Severity of OSAS was quantified by polysomnography. Patients with apnea-hypopnea index (AHI) < 5 were included in the OSAS (-) group (Group 1, n = 22). Subjects with AHI >= 5 were considered as OSAS and classified according to their AHI as mild-to-moderate (AHI >= 5 and AHI < 30) (Group 2, n = 45) and severe (AHI >= 30) OSAS groups (Group 3, n = 40). Conventional M-mode, 2D, and Doppler mitral inflow parameters, tissue Doppler velocities, myocardial peak systolic strain, and strain rate values of various segments were measured and compared between groups. Results: Patients with OSAS displayed impairment of left ventricular diastolic function compared with controls. There were no significant differences between groups regarding parameters reflecting left ventricular systolic function. Myocardial strain analysis demonstrated significant decrement regarding apical right ventricular longitudinal peak systolic strain and strain rate values between groups in relation to the severity of OSAS. Conclusions: Patients with OSAS display a regional pattern of right ventricular dysfunction correlated with the severity of disease.