Obesity is an independent risk factor for the development of heart failure. Several recent studies have found better outcomes of heart failure for obese patients, an observation termed as the "obesity paradox." On the other hand, the negative effect of malnutrition on the evolution of heart failure has also been clearly established.Data from the Minimum Basic Data Set were analyzed for all patients discharged from all the departments of internal medicine in hospitals of the Spanish National Health System between the years 2006 and 2008. The information was limited to those patients with a primary or secondary diagnosis of heart failure. Patients with a diagnosis of obesity or malnutrition were identified. The mortality and readmission indexes of obese and malnourished patients were compared against the subpopulation without these diagnoses.A total of 370,983 heart failure admittances were analyzed, with 41,127 (11.1%) diagnosed with obesity and 4105 (1.1%) with malnutrition. In-hospital global mortality reached 12.9% and the risk of readmission was 16.4%. Obese patients had a lower in-hospital mortality risk (odds ratio [OR]: 0.65, 95% confidence interval [95%CI]: 0.62-0.68) and early readmission risk (OR: 0.81, 95%CI: 0.78-0.83) than nonobese patients. Malnourished patients had a much higher risk of dying while in hospital (OR: 1.83 95%CI: 1.69-1.97) or of being readmitted within 30 days after discharge (OR: 1.39, 95%CI: 1.29-1.51), even after adjusting for possible confounding factors.Obesity in patients admitted for HF substantially reduces in-hospital mortality risk and the possibility of early readmission, whereas malnutrition is associated with important increases in in-hospital mortality and risk of readmission in the 30 days following discharge.