Obesity shortens survival in the general population. In hemodialysis (HD), obesity is associated with improved short-term survival (around 3 years). The discrepancy in the survival of obese patients between HD and the general population may be attributable to survival bias. (Only a small percentage of patients with renal failure survive until HD, and they may have certain survival advantages, including obesity.) Bias is introduced through the mixture of prevalent and incident HD patients in most studies, better nutrition in obese HD patients, malnutrition-inflammation complex syndrome causing weight loss, or other reasons. In studies of peritoneal dialysis (PD), obesity has been associated with decreased patient survival, no noticeable effect on survival, and increased survival. Potential reasons for the differences include bias in the selection of PD for obese patients, effects of race, chronic inflammation in obese PD patients, differences in nutrition and adequacy of PD, adverse effects of the increased PD dose needed to achieve adequate small-solute clearances, differences in body composition, and time discrepancies among risk factors having opposite effects on PD patient survival. Some evidence exists that in the long-term (> 10 years), obesity is a risk factor for death in both HD and PD. Further studies are needed to identify the short- and long-term risks and benefits of obesity in the two dialysis modalities.