Patients with chronic kidney disease (CKD) are at risk to develop acute renal failure (ARF) after open heart surgery. This complication is associated with high morbidity, mortality, and cost. Because the ability to concentrate urine is lost early in the progression of CKD, renal patients kept on fluid restriction prior to surgery may develop severe dehydration, a situation consistently found to be one of the most critical risk factors for postoperative ARF. Our goal was to investigate whether intravenous hydration for 12 h prior to cardiac surgery could prevent acute renal injury in patients with CKD. This is a prospective study in a tertiary cardiac surgery center. Forty-five patients admitted for elective open heart surgery with moderate-to-severe CKD, as evidenced by a quantified glomerular filtration rate less than 45 mL/min, were assigned using a 2/1 randomization process, to either receive an intravenous infusion of half-isotonic saline (1 mL/kg/h) for 12 h before the operation (hydration group, n = 30, 29 men, 64 + 1.7 years old), or to be simply kept on fluid restriction (control group, n = 15, 14 men, 64.2 + 2.8 years old). Groups were not different in clinical and intraoperative variables associated with postoperative renal injury. ARF developed in 8 of 15 (53%) patients in the control group, but in only 9 of the 30 (30%) patients in the hydration group. Four patients in the control group (27%), but no one in the hydration group, required dialysis after the operation (P < 0.01). Peak creatinine and blood urea nitrogen values were two to three times higher in the control group than in the hydration group. Preoperative intravenous hydration may ameliorate renal damage in patients with moderate-to-severe renal insufficiency undergoing cardiac surgery.