Recent studies have indicated that maximal hydration of the transplant recipient can substantially reduce the incidence of acute tubular necrosis (ATN). However, this policy requires invasive hemodynamic monitoring, prolonged mechanical ventilation, and bears the risk of overhydration. In a prospective trial we studied the incidence of ATN in recipients of cadaveric kidneys after restricted fluid infusion (Group 1, N = 21), after restricted fluid infusion along with 250 ml of mannitol 20% (Group 2, N = 19), and after a moderate hydration policy together with 250 ml mannitol 20% (Group 3; N = 21). Donor- and preoperative recipient parameters were comparable in all three groups. The total amount of fluid administered and the incidence of ATN were as follows: Group 1-1059 +/- 371 ml and 43%; Group 2-1548 +/- 622 ml and 53%; and Group 3-2529 +/- 675 ml and 4.8%. The moderate hydration policy in Group 3 resulted in a significantly higher peroperative systolic blood pressure compared to Groups 1 and 2. We did not observe any problems related to overhydration. The reduction of ATN incidence led to a substantial decrease in the number of hemodialysis treatments, radionuclide scans, ultrasound investigations, transplant biopsies, and rejection episodes in the first 3 months after transplantation. It is concluded that moderate fluid administration of 2.5 liters during the transplant procedure together with infusion of 250 ml of mannitol 20% immediately before vessel clamp release reduces the incidence of postoperative ATN below five per cent. The procedure is safe, simple, and does not require invasive hemodynamic monitoring.