BACKGROUND Anemia may have adverse effects in patients with coronary artery disease. However, the benefit of blood transfusion in elderly patients with acute myocardial infarction and various degrees of anemia is uncertain. METHODS We conducted a retrospective study of data on 78,974 Medicare beneficiaries 65 years old or older who were hospitalized with acute myocardial infarction. Patients were categorized according to the hematocrit on admission (5.0 to 24.0 percent, 24.1 to 27.0 percent, 27.1 to 30.0 percent, 30.1 to 33.0 percent, 33.1 to 36.0 percent, 36.1 to 39.0 percent, or 39.1 to 48.0 percent), and data were evaluated to determine whether there was an association between the use of transfusion and 30-day mortality. RESULTS Patients with lower hematocrit values on admission had higher 30-day mortality rates. Blood transfusion was associated with a reduction in 30-day mortality among patients whose hematocrit on admission fell into the categories ranging from 5.0 to 24.0 percent (adjusted odds ratio, 0.22; 95 percent confidence interval, 0.11 to 0.45) to 30.1 to 33.0 percent (adjusted odds ratio, 0.69; 95 percent confidence interval, 0.53 to 0.89). It was not associated with a reduction in 30-day mortality among those whose hematocrit values fell in the higher ranges. In one of seven subgroup analyses (among patients who survived at least two days), transfusion was not associated with a reduction in mortality for patients with hematocrit values of 30.1 percent or higher. CONCLUSIONS Blood transfusion is associated with a lower short-term mortality rate among elderly patients with acute myocardial infarction if the hematocrit on admission is 30.0 percent or lower and may be effective in patients with a hematocrit as high as 33.0 percent on admission.