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Background: Healthy subjects and patients after successful kidney transplantation show a circadian rhythm for glomerular filtration rate and for the glomerular transport of macromolecules. We aimed to evaluate by bioelectrical impedance analysis (BIA) whether body hydration status also follows a circadian rhythm in patients with impaired renal function., Methods: The study was conducted on 28 subjects divided into 3 groups: 8 healthy volunteers, 8 patients affected by chronic kidney disease and 12 end-stage renal disease (ESRD) patients on hemodialysis. During 24 h, 9 BIA measurements were taken in every subject every 180 min., Results: BIA findings demonstrate that normal subjects have a circadian rhythm in hydration status that reaches maximum body water content at night, between 21.00 and 23.00 h. In patients with chronic kidney disease, this rhythm, with maximum at night, is maintained. The rhythm is also present in ESRD patients, if the residual diuresis is at least 500 ml/day, while there is no rhythm when residual diuresis is <300 ml/day., Conclusions: In normal subjects, body hydration status shows a circadian rhythm, which is weakened or lost in oligoanuric patients on dialysis, but partially maintained in subjects with preterminal uremia and in hemodialyzed patients with residual diuresis >500 ml/day., (Copyright 2007 S. Karger AG, Basel.)