Although the adverse health consequences of obesity in the general population have been well documented, recent evidence suggests that obesity is associated with better outcomes in patients with heart failure (HF). Studies of patients with HF that specifically examined the impact of body mass index (BMI) on outcomes have suggested the existence of an "obesity paradox." However, closer examination of these studies raises important questions on the validity of the paradox. First, the diagnosis of HF in obese patients, particularly when made using clinical variables, may not be accurate; the obese patients in these studies may actually be "healthier" than their nonobese comparators. Second, the deleterious effects of cachexia, rather than the salutary ones of obesity, are likely the main reason for the inverse correlation between BMI and HF outcome, especially once the underlying biologic mechanisms behind cachexia and obesity in patients with HF are considered. Furthermore, few studies have specifically examined the more severely obese population (BMI >35 kg/m(2)) when assessing outcomes, and those that have suggest that severely obese patients may have worse outcomes than patients with normal weights or those who are mildly obese. Therefore, a "U-shaped" outcome curve according to BMI for patients with HF may actually exist, in which mortality is greatest in cachectic patients; lower in normal, overweight, and mildly obese patients; but higher again in more severely obese patients. Further prospective studies assessing the impact of more marked degrees of obesity on outcomes in patients with HF are needed to more conclusively determine whether the obesity paradox truly exists.