Clinicians have few data on which to base a decision to transfuse a surgical patient. We reviewed animal and human data to evaluate the effects that anemia and comorbidity have on surgical outcome. Experimental evidence consistently demonstrates increased cardiac output, decreased peripheral vascular resistance, and increased release of oxygen by red blood cells in response to anemia. Normal animals tolerate hemoglobin (Hb) levels down to approximately 5 g/dL. Below this level, cardiac ischemia and decreased ventricular function develop. In animals with experimental coronary artery stenosis, cardiac ischemia develops at Hb levels of 7-10 g/dL. Coexisting left ventricular hypertrophy, use of beta blockers, and hypoxemia also reduce animals' ability to tolerate anemia. The limited information on anemia tolerance of human surgical patients suggests that the presence of cardiac and pulmonary disease should influence transfusion decisions. A higher Hb threshold should be used in patients who have or are at risk of cardiac or pulmonary artery disease.