['In morbidly obese patients (MO), adequate levels of venous return (VR) and left ventricular filling pressures (LVFP) are crucial in order to augment perioperative safety. Rapid weight loss (RWL) preparation with very low calorie diet is commonly used aiming to facilitate bariatric surgery. However, the impact of RWL on VR and LVFP is poorly studied.', 'In this prospective, controlled, single-center study, we hypothesized that RWL-prepared MO prior to bariatric surgery can be hypovolemic (i.e., low VR) and compared MO to lean controls with conventional overnight fasting. Twenty-eight morbidly obese patients were scheduled consecutively for bariatric surgery and 19 lean individuals (control group, CG) for elective general surgery. Preoperative assessment of VR, LVFP, and biventricular heart function was performed by a transthoracic echocardiography (TTE) protocol to all patients in the awake state. Assessment of VR and LVFP was made by inferior vena cava maximal diameter (IVCmax) and inferior vena cava collapsibility index- (IVCCI) derived right atrial pressure estimations.', 'A majority of MO (71.4 %) were hypovolemic vs. 15.8 % of lean controls (p\u2009<\u20090.001, odds ratio\u2009=\u200913.3). IVCmax was shorter in MO than in CG (p\u2009<\u20090.001). IVCCI was higher in MO (62.1\u2009±\u200923 %) vs. controls (42.6\u2009±\u200920.8; p\u2009<\u20090.001). Even left atrium anterior-posterior diameter was shorter in MO compared to CG.'] Is rapid weight loss associated with preoperative hypovolemia in morbidly obese patients?