Patients with chronic kidney disease (stage 5) who undergo hemodialysis treatment have similarities to heart failure patients in that both populations retain fluid frequently and have excessively high mortality. Volume overload in heart failure is associated with worse outcomes. We hypothesized that in hemodialysis patients, greater interdialytic fluid gain is associated with poor all-cause and cardiovascular survival. We examined 2-year (July 2001 to June 2003) mortality in 34,107 hemodialysis patients across the United States who had an average weight gain of at least 0.5 kg above their end-dialysis dry weight by the time the subsequent hemodialysis treatment started. The 3-month averaged interdialytic weight gain was divided into 8 categories of 0.5-kg increments (up to > or =4.0 kg). Eighty-six percent of patients gained >1.5 kg between 2 dialysis sessions. In unadjusted analyses, higher weight gain was associated with better nutritional status (higher protein intake, serum albumin, and body mass index) and tended to be linked to greater survival. However, after multivariate adjustment for demographics (case mix) and surrogates of malnutrition-inflammation complex, higher weight-gain increments were associated with increased risk of all-cause and cardiovascular death. The hazard ratios (95% confidence intervals) of cardiovascular death for weight gain <1.0 kg and > or =4.0 kg (compared with 1.5 to 2.0 kg as the reference) were 0.67 (0.58 to 0.76) and 1.25 (1.12 to 1.39), respectively. In hemodialysis patients, greater fluid retention between 2 subsequent hemodialysis treatment sessions is associated with higher risk of all-cause and cardiovascular death. The mechanisms by which fluid retention influences cardiovascular survival in hemodialysis may be similar to those in patients with heart failure and warrant further research.