Despite recent therapeutic advances, significant residual risk for in-hospital mortality persists among patients admitted with acute myocardial infarction (MI). Low levels of high-density lipoprotein cholesterol (HDL-C), a known independent predictor of increased cardiovascular events, may be an important modulator of heightened risk after acute MI. We evaluated admission HDL-C levels among 98,276 patients with non-ST elevation myocardial infarction with acute MI from the Acute Coronary Treatment and Intervention Outcomes Network Registry–Get With the Guidelines (ACTION Registry–GWTG) program who were enrolled from 490 United States hospitals from January 2007 to December 2010. Clinical characteristics, treatments, atherosclerotic burden, and in-hospital outcomes were analyzed by quartiles of admission HDL-C (Q1: 10 to 30 mg/dl; Q2: 30.1 to 36.9 mg/dl; Q3: 37 to 45 mg/dl; and Q4: 45.1 to 100 mg/dl). Logistic regression was used to explore the relation among HDL-C quartiles, coronary artery disease severity, and in-hospital mortality. Almost half of the patients with acute MI had low admission levels of HDL-C (less than the median 36.9 mg/dl). Such patients were younger, more often men, white, obese, diabetic, smokers, and had higher rates of previous cardiovascular events. After multivariate adjustment, patients with low HDL-C levels had greater extent of severe angiographic multivessel coronary narrowings and higher mortality. Among the 26% of patients in the lowest HDL-C quartile (≤30 mg/dl), there was a 16% greater risk of in-hospital mortality compared with patients in the highest HDL-C quartile (p = 0.012). In conclusion, low levels of HDL-C were common in patients admitted with acute MI and were associated with more extensive angiographic coronary disease. Very low levels of admission HDL-C were observed in one-quarter of patients and associated with significantly higher in-hospital mortality.