Abstract Background and Aims Altered fluid status and inadequate fluid management, independently and concurrently with depletion of lean mass adversely affect patients’ quality of life, outcomes and prognosis of hemodialysis (HD) patients, both standard (HD) and Hemodiafiltration on-line (HDFOL). Method We conducted a population-based on 470 patients, retrospective longitudinal cohort study, to determine the physical characteristics of bioimpedance measurements and indexes obtained by BIVA on maintenance HD and HDFOL patients, and to analyze the hydration status of the extra and intracellular compartments. The Propensity Score Matching (PSM) procedure, matches case records with similar control records. It first runs a logistic regression with the case/control group variable as the dependent variable. Then it selects a match for each case from the control group based on the PSM from the logistic regression. Euhydrated and Overhydrated groups were macheted separately. Results see Table 1, Figure 1 and 2. Conclusion 1) Compared to standard HD, the HDFOL technique maintains a better water balance between the intracellular and extracellular spaces, especially when the patient is euhydrated. In overhydrated patients, although the ECW/ICW ratio is high, it is lower in patients on HDFOL. 2) The stability of cell membranes in HDFOL is better than in HD. 3) For an adequate assessment of any of the parameters measured directly or derived from the BIA, it must be a “sine qua non” condition that the patient is in a state of euhydration. 4) In hemodialysis patients, the phase angle, must first be considered as a marker of hydration status, and then as a marker of nutritional status.