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Background: Prior studies on chronic systolic heart failure (HF) have demonstrated that body mass index (BMI) is inversely associated with mortality, the so-called obesity paradox. The aim of this study was to determine whether BMI influences the mortality risk in acute decompensated HF, a subject not previously studied., Methods: The Acute Decompensated Heart Failure National Registry was analyzed for acute HF hospitalizations in 263 hospitals in the United States from October 2001 through December 2004. Patients with documented height and weight were divided into BMI (measured in kilograms per square meter) quartiles. Inhospital mortality by BMI quartile for all the patients and for those with reduced (n = 43,255) and preserved (n = 37,901) systolic function was assessed., Results: Body mass index quartiles in the 108,927 hospitalizations were QI (16.0-23.6 kg/m2), QII (23.7-27.7 kg/m2), QIII (27.8-33.3 kg/m2), and QIV (33.4-60.0 kg/m2). Patients in the higher BMI quartiles were younger, had more diabetes, and had a higher left ventricular ejection fraction. Inhospital mortality rates decreased in a near-linear fashion across successively higher BMI quartiles. After adjustments for age, sex, blood urea nitrogen, blood pressure, creatinine, sodium, heart rate, and dyspnea at rest, BMI quartile still predicted mortality risk. For every 5-U increase in BMI, the odds of risk-adjusted mortality was 10% lower (95% CI 0.88-0.93, P < .0001)., Conclusions: In this cohort of hospitalized patients with HF, higher BMI was associated with lower inhospital mortality risk. The relationship between BMI and adverse outcomes in HF appears to be complex and deserving of further study.