INTRODUCTION The early detection of critically ill patients together with the rapid initiation of effective treatment in emergency departments(ED) increase the survival rates. AIM This study investigated whether a correlation exists between haemodynamic parameters of critically ill patients and the diameter of the inferior vena cava (IVC). MATERIALS AND METHODS A cross-sectional study was performed included patients aged ≥18 years with an unstable haemodynamic and/or respiratory status who were referred to the ED for non-traumatic issues. IVC diameters were measured by ultrasound (US) and then central venous pressures (CVP) were measured. Anteroposterior (AP) and mediolateral (ML) diameters of the IVC, both in the inspirium (IAP, IML) and expirium (EAP, EML), were measured by US. RESULTS 102 patients were evaluated with a median age of 59. The relationship between the diameters of IVC and CVP was evaluated and significant correlation was found in IAP, EAP according to CVP values (p<0.001). ROC analyses were performed and significant relationship was found between the EAP diameter with haemoglobin (Hmg), haemotocrit (Hct), and central venous oxygen saturation (ScvO2) and also significant correlation was detected between the IAP diameter and white blood cell (WBC). DISCUSSION We detected significant correlation between the CVP and the IVC diameter in our study compatible with recent studies besides, significant correlation was found between the diameter of the IVC and CVP values as well as between the EAP diameter and Hmg, Hct, ScvO2 levels. CONCLUSION Measurement of IVC diameters, especially EAP may be useful at the monitoring of critically ill patients in ED.