To investigate the impact of coffee consumption on fluid balance, 12 healthy volunteers were supplied with a standardized diet for 2 days after having abstained from consumption of methylxanthines for 5 days. During the first day, fluid requirement was met by mineral water. On the following day the same amount of fluid was supplied and the mineral water was in part replaced by 6 cups of coffee containing 642 mg of caffeine. This led to an increase in 24-hour urine excretion of 753 ± 532 ml (p < 0.001), a corresponding negative fluid balance and a concomitant decrease in body weight of 0.7 ± 0.4 kg (p < 0.001). Total body water as measured with bioelectrical impedance analysis decreased by 1.1 ± 1.2 kg or 2.7% (p < 0.01). Urinary excretion of sodium and potassium was elevated by 80 ± 62 mmol or 66% (p < 0.01) and 14 ± 12 mmol or 28% (p < 0.01), respectively.