Background: An accurate assessment of fluid status in children on dialysis is particularly difficult in growing children, because of continous changes in weight. Inferior vena cava (IVC) measurements have been suggested as a reliable and easy method for evaluation of hydration in adult HD patients. Underestimating dry weight during haemodialysis can cause hypovolemia, hypotension, nausea, headaches, and muscular cramps, while overestimating it can result in chronic fluid overload, hypertension, and heart failure. Clinical measures (blood pressure, heart rate, oedema, and venous congestion) are frequently unreliable and also affected by causes other than hydration. The measurement of inferior vena cava diameter (IVCD) by echocardiography has been suggested as a reliable, bedside method for evaluating hydration in adult haemodialysis patients. Invasive methods for fluid status assessment, such as measurement of central venous pressure, are not easily applicable. Methods: In this upcoming cohort research IVC echocardiography was performed within 15 minutes before and after dialysis in our juvenile tertiary pediatrics nephrology center on a total of 60 children undergoing dialysis for end-stage renal disease (ESRD) or acute kidney injury (AKI). The ultrasonography alterations before and after dialysis were evaluated, and a correlation analysis was performed on these results. Results: IVC collapsibility index increased in 98.3% of patients after dialysis. In the individuals who were evaluated both before and after dialysis, there is a statistically significant relationship between the kind of dialysis (acute or chronic) and IVC. After dialysis, the IVC changes statistically significantly (p0.001 in chronic cases and 0.003 in acute cases). Conclusion: Echocardiography IVC collapsibility assessment has a significant diagnostic and prognostic value for volume status assessment in HD children, that is easily and safety performed without complications.