BACKGORUND: To choose optimal potassium free solutions in chronic renal failure patients, we studied the effects of 0.45% or 0.9% saline solutions on the serum sodium concentrations of patients undergoing kidney transplantation surgery. METHODS: Sixty two patients, ASA physical status iii iV, undergoing kidney transplantation surgery, were allocated to receive either 0.45% saline (n = 26) or 0.9% saline solutions (n = 36). The patients received 2 L of 0.45% or 0.9% saline solutions with a CVP of 10 mmHg prior to a renal vein anastomosis. Serum sodium and potassium concentrations were measured before (Na0, K0) and after the administration of 2 L of 0.45% or 0.9% saline solutions (Na2, K2), respectively. RESULTS: Serum sodium concentrations decreased after the administration of 0.45% saline solutions, and after administrating 2 L of 0.9% saline, the serum sodium concentrations increased in patients with baseline serum sodium concentrations lower than 136 mEq/L. When we administered 0.9% saline, which had a sodium concentration lower than the serum sodium concentration, the serum sodium concentration measured after 2 L of 0.9% saline infusion proportionally decreased in patients with baseline serum sodium concentration greater than 136 mEq/L (P<0.001). CONCLUSiONS: For ideal fluid selection in chronic renal failure patients, this study suggests a 0.45% saline solution for chronic renal failure patients with a baseline serum sodium concentration below the normal range and a 0.9% saline solution for patients with a baseline sodium concentration within the normal range.