Vesicoureteral reflux (VUR) is a common abnormality of the urinary tract in childhood.As urine enters the ureters and renal pelvis during voiding in vesicoureteral reflux (VUR), we hypothesized that change in body water composition before and after voiding may be less different in children with VUR.Patients were grouped as those with VUR (Group 1) and without VUR (Group 2). Bioelectric impedance analysis was performed before and after voiding, and third space fluid (TSF) (L), percent of total body fluid (TBF%), extracellular fluid (ECF%), and intracellular fluid (ICF%) were recorded. After change of TSF, TBF, ECF, ICF (ΔTSF, ΔTBF%, ΔECF%, ΔICF%), urine volume (mL), and urine volume/body weight (mL/kg) were calculated. Groups 1 and 2 were compared for these parameters. In addition, pre- and post-voiding body fluid values were compared in each group.TBF%, ECF%, ICF%, and TSF in both pre- and post-voiding states and ΔTBF%, ΔECF%, ΔICF%, and ΔTSF after voiding were not different between groups. However, while post-voiding TBF%, ECF% was significantly decreased in Group 1 (64.5 ± 8.1 vs 63.7 ± 7.2, P = 0.013 for TBF%), there was not post-voiding change in TSF in the same group. On the other hand, there was also a significant TSF decrease in Group 2.Bladder and ureter can be considered as the third space. Thus, we think that BIA has been useful in discriminating children with VUR as there was no decreased in patients with VUR, although there was decreased TSF in patients without VUR. However, further studies are needed to increase the accuracy of this hypothesis.