Background Assessment of hydration status in patients with chronic kidney failure treated by dialysis is crucial for clinical management decisions. Dilution techniques are considered the gold standard for measurement of body fluid volumes, but they are unfit for day-to-day care. Multifrequency bioimpedance has been shown to be of help in clinical practice in adults and its use in children and adolescents has been advocated. We investigated whether application of multifrequency bioimpedance is appropriate for total-body water (TBW) and extracellular water (ECW) measurement in children and adolescents on dialysis therapy. Study Design A study of diagnostic test accuracy. Setting & Participants 16 young dialysis patients (before a hemodialysis session or after peritoneal dialysis treatment) from the Ca’ Granda Ospedale Maggiore Policlinico, Milan, Italy, and the Emma Children's Hospital−Academic Medical Center, Amsterdam, the Netherlands. Index Test TBW and ECW volumes assessed by multifrequency bioimpedance. Reference Tests TBW and ECW volumes measured by deuterium and bromide dilution, respectively. Results Mean TBW volumes determined by multifrequency bioimpedance and deuterium dilution were 19.2 ± 8.7 (SD) and 19.3 ± 8.3 L, respectively; Bland-Altman analysis showed a mean bias between the 2 methods of −0.09 (95% limits of agreement, −2.1 to 1.9) L. Mean ECW volumes were 8.9 ± 4.0 and 8.3 ± 3.3 L measured by multifrequency bioimpedance and bromide dilution, respectively; mean bias between the 2 ECW measurements was +0.6 (95% limits of agreement, −2.3 to 3.5). Limitations Participants ingested the deuterated water at home without direct supervision by investigators, small number of patients, repeated measurements in individual patients were not performed. Conclusions Multifrequency bioimpedance measurements were unbiased but imprecise in comparison to dilution techniques. We conclude that multifrequency bioimpedance measurements cannot precisely estimate TBW and ECW in children receiving dialysis.