Cardiac surgery can either induce acute renal failure or improve GFR by improving the cardiac performance. In order to study renal function changes after elective cardiac surgery (CS) with cardiopulmonary bypass (CPBP), 21 patients undergoing valvular CS (VCS) or coronary artery bypass (CAB) were prospectively evaluated in three time periods: before, 24 hours after surgery and 48 hours after surgery. Patients were divided in 2 groups according to the GFR percent change in comparison to the baseline value found 24 hours after CS (deltaGFR24): Group 1, deltaGFR24 decrease higher than 20% (n = 11) and Group 2, deltaGFR24 decrease < or = 20% or deltaGFR24 increase (n = 10). In Group 1, 73% of the patients underwent VCS (p = 0.05 vs. Group 2) and all of them had previous VCS in sharp contrast with Group 2, where none of the patients had previous CS (p = 0.006). Patients in Group I required more volume replacement than Group 2 during the first 24 hours after CS: 2,699+/-704 mL versus 217+/-603 mL respectively, p = 0.019. Despite similar baseline GFR, Group 1 presented lower GFR 24 hours after CS when compared to Group 2 (39+/-5 versus 75+/-8 mL/(min x 1.73m2), p = 0.001) and a significantly different deltaGFR 48 hours after CS as compared to Group 2 (-21+/-11 versus +88+/-36%, p<0.01). Baseline sodium fractional excretion (FENa) in Group 1 was lower than in Group 2 (0.27+/-0.04 versus 0.70+/-0.12%, p = 0.01). No changes were observed after CS in urinary osmolality (Uosm) and urinary pH (UpH) in both groups. The deltaGFR24 showed positive correlation with baseline FENa (r = 0.44 p = 0.04) and negative correlation with volume balance during the first 24h after CS (r = -0.63, p = 0.007). More patients in Group 1 required nitroprusside than in Group 2 (66% vs. 14%, p = 0.04). Anesthesia time was shorter in Group 1 as compared to Group 2: 323+/-21 vs. 395+/-26 min, p = 0.04. No significant hemolysis occurred during CS in either group. There were no differences in age, gender, CPBP time, need for dopamine and/or dobutamine between the two groups. In conclusion, patients who presented GFR decrease after CS underwent VCS more frequently, had more prevalence of previous CS, presented lower baseline FENa, required more volume infusion and more nitroprusside use. On the other hand, no tubular dysfunction was detected in the early follow-up of CS. These results suggest that the observed renal function changes should be the result of an appropriated renal response to a low effective blood volume. In fact, a low baseline FENa anticipated a GFR decrease in these patients. Consistently, CAB patients that usually improve their cardiac output after surgery showed a clear GFR improvement.