Echocardiograms were taken from the parasternal long axis view during nocturnal sleep in ten patients diagnosed with OSAS. A table designed to support the echocardiographic probe prevented significant sleep disturbances during monitoring and allowed continuous data collection with and without nasal CPAP administration. In five of ten patients, there was before CPAP treatment a diastolic LSIVS during NREM sleep, inducing a flattening of the left ventricle. Arterial blood pressure recordings showed pulsus paradoxus when LSIVS was occurring. Nasal CPAP led to normal, unobstructed breathing, significant decrease in Pes nadir and disappearance of LSIVS and pulsus paradoxus. Increase in left ventricular afterload and increase in total peripheral resistance could lead to hypertrophy and hypertension in some OSAS patients. The presence of pulsus paradoxus in OSAS indicates a marked increase in Pes nadir, and its disappearance with nasal CPAP may be one of the signs of effective treatment of OSAS.