Hypernatremia was observed in five recipients of renal homografts during the first postoperative week. The peak serum levels of sodium varied between 152 and 158 mEq/liter. The postoperative diuresis was associated with sodium concentration in the urine, consistently lower than that in the extracellular water; moderate urinary hypertonicity, with urea being the main urinary solute; and urea excretion exceeding 60% of its filtered load in most instances. The inability of elaborate urine with sodium concentration equal or higher than that in the serum, possibly related to osmotic diuresis and/or altered renal hemodynamics, appears to be the primary cause responsible for the development of hypernatremia in these patients.