Background and objective: Low hematocrit level and transfusion may coexist during cardiopulmonary bypass and the actual impact of one on the outcome parameters may be counfounded or masked by the other. This study aims to determine the impact of the lowest hematocrit level during cardiopulmonary bypass on outcome parameters in nontransfused patients. Methods: Two thousand six hundred and thirty- two consecutive patients who underwent isolated coronary artery bypass graft (CABG) surgery with cardiopulmonary bypass were evaluated prospectively:1854 (70.4%) patients who did not receive any red blood cells during hospital stay were included in the study. Perioperative data and outcome parameters were recorded. Outcomes were evaluated in 2 groups according to the lowest level of hematocrit (>21%: high hematocrit group, n=1680, (91.6%) and <= 21%: low hematocrit group, n=174, (9.4%)) during cardiopulmonary bypass. Results: Overall mean lowest hematocrit level of patients was 27.7 +/- 4.4% (19.7 +/- 1.9% in the low hematocrit group, 28.5 +/- 4.1% in the high hematocrit group). The comparison of outcome parameters regarding the time on ventilator, duration of intensive care unit stay, intensive care unit re- admission, hospital re- admission, reoperation for bleeding or tamponade, low cardiac output, postoperative atrial fibrillation, stroke, creatinine level at hospital discharge, new onset renal failure, mediastinitis, pulmonary complication and mortality rates were similar in both groups. Conclusions: Our findings suggest that a lowest hematocrit level of <= 21% during cardiopulmonary bypass has no adverse impact on outcome after isolated coronary surgery in non- transfused patients.