Introduction: Untreated obstructive sleep apnea (OSA) is associated with a high cardiovascular mortality and morbidity, probably related to the action of intermittent hypoxia on cardiac and vascular function; however, detailed mechanisms and methods for early detection are still debatable. Methods: 84 subjects (53 ± 11 years, 20 women) were stu died: 42 patients with moderate-to-severe OSA (mean apnea-hypopnea index 44 ± 21), before starting cPAP treatment, without known cardiovascular disease or risk factors, and 42 age- and sex-matched normal subjects. LV systolic function was assessed by ejection fraction (EF), mean basal systolic velocity (S\’), and global longitudinal strain (GLS); RV systolic function by tricuspid annular systolic velocity (RV-S\’), global strain (RVGS), and systolic pulmonary artery pressure (SPAP). Arterial function was assessed by intima-media thickness (IMT) and “e-tracking” for measuring the elastic modulus (Ep) and the augmentation index (AIx). NT pro-BNP was determined. Results: Parameters of LV and RV systolic function were significantly lower in patients with OSA (EF 57 ± 6% in OSA vs 67 ± 8% control, p ≤ 0.01, S\’ 5.1 ± 1.1 cm/s in OSA, vs 6.8 ± 1.2 cm/s control, p ≤ 0.01, GLS -14.2 ± 6.8% in OSA vs -19.3 ± 2.6% control, p ≤ 0.01, RV-S\’ 9.4 ± 2.8 cm/s in OSA vs 11.7 ± 1.8 cm/s control, p = 0.03, RVGS -16.4 ± 7.0% in OSA vs -20.7 ± 8.6 control, p = 0.03), with increased NT pro-BNP (63 ± 6 pg/ml in OSA vs 35 ± 2 pg/ml control, p = 0.03) and arterial stiff ness (IMT 0.9 ± 0.2 mm in OSA vs 0.7 ± 0.1 mm control, p ≤ 0.01, Ep 134 ± 47 in OSA vs 95 ± 50 control, p ≤ 0.01, AIx 17.1 ± 22.0% in OSA vs 8 ± 13.4%, p ≤ 0.01). SPAP was normal, but signifi cantly higher in OSA (22.9 ± 10.8 mm Hg in OSA vs 18.2 ± 8.1 mm Hg control, p = 0.04). Conclusions: Patients with moderate-to-severe OSA have subclinical systolic biventricular dysfunction associated with increased arterial stiff ness. Early diagnosis of these changes by new echo methods should be implemented for preventive actions.