Background: Hospital-acquired anemia (HAA) is common during acute myocardial infarction (AMI) and associated with higher long-term mortality. The relationship between HAA and adverse in-hospital outcomes may be particularly relevant to hospitals' efforts to implement prevention programs, but the association between HAA and in-hospital mortality is unclear. Methods: We studied 17,676 patients with AMI with normal admission hemoglobin level who did not undergo bypass surgery. Hospital-acquired anemia was defined as development of new anemia during hospitalization (based on nadir hemoglobin) using age-, gender-, and race-specific criteria. In-hospital mortality of patients with mild (hemoglobin level less than HAA threshold but >11 g/dL), moderate (hemoglobin level 9-11 g/dL), and severe HAA (hemoglobin level, < 9 g/dL) was compared with those without HAA using hierarchical logistic regression, adjusting for site and potential confounders. Results: Hospital-acquired anemia developed in 10,166 patients (57.5%); 6,615 (37.4%) had mild; 2,740 (15.5%), moderate; and 811 (4.6%), severe HAA. In-hospital mortality was higher in patients with HAA and increased with HAA severity (no HAA 266 [3.5%], mild HAA 260 [3.9%], moderate HAA 222 [8.1%], and severe HAA 148 [18.3%], P <.001). The adjusted odds of in-hospital death were greater in patients with moderate (odds ratio 1.38, 95% CI 1.10-1.73) and severe HAA (3.39, 95% CI 2.59-4.44) versus no HAA. Conclusions: Moderate and severe HAAs are independently associated with higher in-hospital mortality during AMI. Studies are needed to determine whether HAA is preventable and if preventing HAA improves outcomes.