It has been reported that sodium intake can be estimated in dialysis patients by the increment in the body sodium pool from the end of a dialysis session to the beginning of the following one. To verify the reliability of this method we compared the sodium intake, estimated by the interdialytic changes in plasma sodium concentration (C) and body water volume (V), to sodium removal during three consecutive sessions. For this purpose we investigated 9 nondiabetic patients, 5 females and 4 males, under chronic hemofiltration treatment. Sodium intake was estimated by the formula (C<sub>pre</sub> V<sub>pre</sub>) – (C<sub>post</sub> V<sub>post</sub>) using a flame photometer and electrical bioimpedance to determine the plasma sodium concentration and total body water, respectively. Sodium removal was calculated by the difference between sodium loss into the ultrafiltrate and sodium gain with the reinfusion fluid. The mean values of sodium intake calculated during the three intervals corresponded with the sodium losses measured during the three hemofiltration sessions in each patient (338 ± 55 vs. 329 ± 67 mEq; r = 0.92, p < 0.0001). A direct relationship was also observed between sodium intake and both interdialytic body weight increase (r = 0.89, p < 0.0001) and fluid loss during the sessions (r = 0.88, p < 0.0001). This data demonstrates that sodium intake can be properly estimated by the interdialytic changes in body water and plasma sodium concentrations. They also suggest that fluid intake may be influenced by sodium consumption and that sodium intake monitoring could be useful for the control of excessive interdialytic fluid gain.