OBJECTIVES: This study sought to define the relationship between body mass index (BMI) and mortality in heart failure (HF) across the world and to identify specific groups in whom BMI may differentially mediate risk. BACKGROUND: Obesity is associated with incident HF, but it is paradoxically associated with better prognosis during chronic HF. METHODS: We studied 6,142 patients with acute decompensated HF from 12 prospective observational cohorts followed-up across 4 continents. Primary outcome was all-cause mortality. Cox proportional hazards models and net reclassification index described associations of BMI with all-cause mortality. RESULTS: Normal-weight patients (BMI 18.5 to 25 kg/m(2)) were older with more advanced HF and lower cardiometabolic risk. Despite worldwide heterogeneity in clinical features across obesity categories, a higher BMI remained associated with decreased 30-day and 1-year mortality (11% decrease at 30 days; 9% decrease at 1 year per 5 kg/m(2); p < 0.05), after adjustment for clinical risk. The BMI obtained at index admission provided effective 1-year risk reclassification beyond current markers of clinical risk (net reclassification index 0.119, p < 0.001). Notably, the "protective" association of BMI with mortality was confined to persons with older age (>75 years; hazard ratio [HR]: 0.82; p = 0.006), decreased cardiac function (ejection fraction <50%; HR: 0.85; p < 0.001), no diabetes (HR: 0.86; p < 0.001), and de novo HF (HR: 0.89; p = 0.004). CONCLUSIONS: A lower BMI is associated with age, disease severity, and a higher risk of death in acute decompensated HF. The "obesity paradox" is confined to older persons, with decreased cardiac function, less cardiometabolic illness, and recent-onset HF, suggesting that aging, HF severity/chronicity, and metabolism may explain the obesity paradox.