BACKGROUND: Recent studies have shown that blood transfusions, and especially red blood cells, are associated with potential adverse outcomes. This study was designed to investigate the effects of red blood cell transfusion according to the hematocrit on the clinical outcomes after cardiac surgery. METHODS: The 433 patients who were undergoing cardiac surgery were randomized to two groups. One group was transfused red blood cells with a hematocrit of 20%, and the other group was transfused red blood cells with a hematocrit of 20~25%. The amounts of intraoperative and postoperative transfusion and various parameters of the clinical outcomes were checked. RESULTS: In the hematocrit <20% group, the amount of infused crystalloid during operation was larger than that of the hematocrit >20% group, and the postoperative hemoglobin and hematocrit were lower than that of the hematocrit >20% group. But there were no differences of the amounts of intraoperative and postoperative transfusion, the use of inotropics, the platelet count, the prothrombin time (PT), the activated partial thromboplastin time (aPTT), the levels of aspartate aminotransferase (AST), the levels of alanine aminotransferase (ALT), blood urea nitrogen (BUN), serum creatinine (Cr), brain natriuretic peptide (BNP) and creatine kinase MB (CK-MB), the extubation time and the ICU stay time between the two groups. CONCLUSION: A hematocrit lower than 20% was tolerated by the cardiac surgical patients and it was not related to the postoperative morbidity and outcomes.