Introduction To identify the impact of storage time and amount of transfused red blood cell units on renal function. Methods Consecutive transfused patients (n=492), undergoing cardiac surgery at a single centre and receiving at least one red blood cell unit, were pooled in different groups depending on storage time and amount of transfusion. Results Altogether 2,133 red blood cell units were transfused (mean age 21.87 days). Pre- and intraoperative data were similar between groups. Postoperative serum creatinine (p<0.01), glomerular filtration rate (p<0.01), and urea (p<0.01) showed a significant correlation with the amount of transfused red blood cell units, but not with storage time. Acute kidney insufficiency (creatinine values greater than 2.0 mg/dl or a duplication of the preoperative value) developed in 29% of patients and was associated with red blood cell mean age (p=0.042), absolute age (p=0.028), and amount of transfused (p<0.01) units. Acute kidney failure requiring renal replacement therapy occurred in 9.6% of patients and was associated with the amount of transfusion (p<0.01). Conclusions Worsening of renal function after cardiac surgery is associated with storage time and amount of transfused red blood cell units. Acute kidney insufficiency was defined as serum creatinine values greater than 2.0 mg/dl or a duplication of the preoperative value (baseline). Acute kidney failure was defined as becoming dependent upon dialysis.