Despite significant technical improvements (bicarbonate dialysate, volumetric ultrafiltration control) high intradialytic ultrafiltration is troublesome in children, specially in the proportion of patients presenting a normal or low blood pressure even with overweight. We used, in this group of children (overhydratated without vascular repercution) a modelling of both sodium and ultrafiltration during dialysis, in order to achieve dry body weight without increasing session time despite hypotension risks. The usefulness and practicability of sodium and ultrafiltration modelling together during dialysis in children is analysed in a short time study (for plasma volume changes calculation) and in a long term follow up study over a year (for clinical tolerance). Today, we reserve this form of dialysis only for a single session needed by overhydratation (more than 5% of dry body weight) in order to achieve dry body weight maintaining dialysis session time constant without increasing side effects (hypotension).