Measurement of the inferior of vena cava (IVC) diameter by ultrasonography (US) has been shown to be useful for assessing dry weight (DW) in hemodialysis (HD) patients. We have previously observed some cases in which the IVC diameter differed depending on whether measurements were obtained from sagittal or cross-sectional images. Thus, in the present study we introduce a new concept, the flat ratio, to define the magnitude of IVC deflation. The flat ratio of the IVC (F-IVC) is determined from cross-sectional ultrasonographic images of the IVC. The present study showed that F-IVC is significantly correlated with changes in body weight (ΔBW(%)). Similarly, the change in F-IVC (ΔF-IVC) during HD is correlated with ΔBW(%) but not with changes in systolic or diastolic blood pressure during HD. The results of the present study suggest that F-IVC measurement can serve as a useful tool for DW assessment.