Background Anemia and red blood cell (RBC) transfusions are both associated with morbidity and mortality after cardiac surgery. Patients with the lowest hematocrit (HCT) values during cardiopulmonary bypass (CPB) are the most likely to receive a transfusion, which results in a double-negative exposure. We aimed to clarify the effects of anemia, transfusion, and their combination to identify which imposes the greatest risk of end-organ dysfunction and mortality. Methods From November 1, 2004, to November 1, 2009, 7942 patients underwent procedures requiring CPB and did not receive intraoperative or postoperative RBC transfusion, and 1202 received intraoperative RBC transfusion alone. They were divided into 4 groups: intraoperative nadir HCT ≥25% without RBC transfusion, ≥25% with RBC transfusion, <25% without RBC transfusion, and <25% with RBC transfusion. The relationship among HCT, RBC, and outcomes was studied using generalized propensity-score analysis. Outcomes included estimated glomerular filtration rate (eGFR), troponin, ventilatory support time, length of stay, and mortality. Results After risk adjustment, comparison of all 4 groups showed that double exposure to anemia (HCT <25%) and RBC transfusion was associated with the highest risk: lowest eGFR (P =.008), highest troponin values (P =.01), longest ventilator requirement (P <.001), longest length of stay (P <.001), and highest mortality (P =.007). Single exposure to either HCT <25% or RBC transfusion alone was associated with the next risk category, and the lowest morbidity risk was associated with neither exposure. Conclusions Although single exposure to anemia or RBC transfusion alone was associated with risk, it was generally lower than that of anemia and RBC exposure in combination.