According to the urea kinetic model it is considered that the urea distribution volume (V) is that of body water, and that it is distributed in only one compartment. Since the V value is difficult to measure, it is normal to use 58% of body weight, in spite of the fact that it may range from 35 to 75%. In this study, we have calculated the value of V by using an accurate method based on the total elimination of urea from the dialysate. We have studied the V, and also whether the different dialysis characteristics modify it. Thirty-five patients were included in this study, 19 men and 16 women, under a chronic hemodialysis programme. The dialysate was collected in a graduated tank, and the concentration of urea in plasma and in dialysate were determined every hour. Every patient received six dialysis sessions, changing the blood flow (250 or 350 ml/min), the ultrafiltration (0.5 or 1.51/h), membrane (cuprophane or polyacrylonitrile) and/or buffer (bicarbonate or acetate). At the end of the hemodialysis session, the V value ranged from 43 to 72% of body weight; nevertheless, this value was practically constant in every patient. The V value gradually increased throughout the dialysis session, 42.1 ± 6.9% of body weight in the first hour, 50.7 ± 7.5% in the second hour and 55.7 ± 7.9% at the end of the dialysis session. The change of blood flow, ultrafiltration, membrane or buffer did not alter the results. The V value was significantly higher in men in comparison with women, 60.0 ± 6.6% vs. 50.5 ± 5.9% of body weight (p < 0.