Background: decisions regarding fluid therapy, whether in the operating theatre, intensive care unit, emergency department, are among the most challenging and important tasks that clinicians face on a daily basis. Specifically, almost all clinicians would agree that both hypovolaemia and volume overload increase the morbidity and mortality of patients. The therapeutic goal of fluid administration is to increase preload, or the stressed venous volume, leading to an in­creased stroke volume and cardiac output. However, studies of patients with acute illness or hypotensive patients in the intensive care unit consistently demonstrate that approximately 50% of fluid boluses fail to achieve the intended effect of increasing car­diac output. Aim of the Work: this study was done to evaluate the correlation between central venous pressure (CVP) measurements and ultrasound measurements of the inferior vena cava diameter, and collapsibility index. The secondary aim was to evaluate the value of ultrasound as a noninvasive tool in assessment of intravascular volume status and fluid responsiveness in critically ill intensive care unit patients. Patients and Methods: after obtaining the approval of the Al-Azhar University Ethical Committee and written informed consent, 50 patients aged 30-60 years of either sex, ASA I-III admitted in the ICU of Al-Azhar teaching hospitals who had a functioning central venous catheter inserted for any clinical indication, were involved in this single blinded correlational study.  Hemodynamic parameters were monitored continuously including heart rate and non-invasive mean arterial blood pressure. CVP measurements were taken with the patient in the supine position. Clinical assessment was done for signs of hypovolemia like hypotension, tachycardia, prolongation of capillary refill: >3 seconds, acidosis, increased serum lactate more than 2 mmol/L or loss of skin turgor. Results: in our study, there was a significant correlation between CVP and the two studied ultrasound parameters, IVC CI and IVCdmax. Analysis of the receiver operating characteristic curve ROC showed that inferior vena cava collapsibility index (IVC CI) had the most favorable performance of the two ultrasound parameters in predicting CVP < 10 cm H2O.  As regards prediction of fluid responsiveness, analysis of the ROC showed a better diagnostic accuracy of IVC collapsibility index and IVC diameter for predicting fluid responsiveness. Conclusion: ultrasound of the inferior vena cava may be used as a feasible non-invasive, rapid and simple adjuvant method to assess the intravascular volume and guide fluid responsiveness in critically ill intensive care unit patients, inferior vena cava collapsibility index may be used to predict low central venous pressure and predict fluid responsiveness.