BACKGROUND: Hyperphosphatemia is an independent risk factor for all-cause and cardiovascular mortality in hemodialysis (HD) patients. Phosphate control often is unsuccessful using conventional dialysis therapies. STUDY DESIGN: Short-term analysis of a secondary outcome of an ongoing randomized controlled trial. SETTING & PARTICIPANTS: 493 (84%) consecutive patients from 589 patients included in the Convective Transport Study (CONTRAST) by January 2009 from 26 centers in 3 countries. INTERVENTION: Online hemodiafiltration (HDF) versus continuation of low-flux HD. OUTCOMES: Differences in change from baseline to 6 months in phosphate levels and proportion of patients reaching phosphate treatment targets (phosphate < or = 5.5 mg/dL). MEASUREMENTS: Phosphate, use of phosphate-binding agents, and proportion of patients achieving treatment targets at baseline, 3 months, and 6 months. RESULTS: Phosphate levels decreased from 5.18 +/- 0.10 (SE) mg/dL at baseline to 4.87 +/- 0.10 mg/dL at 6 months in HDF patients (P < 0.001) and were stable in HD patients (5.10 +/- 0.10 mg/dL at baseline and 5.03 +/- 0.10 mg/dL after 6 months; P = 0.5). The difference in change in phosphate levels between HD and HDF patients (B = -0.24; 95% CI, -0.52 to 0.03; P = 0.08) increased after adjustment for phosphate-binder use (B = -0.36; 95% CI, -0.65 to -0.06; P = 0.02). The proportion of patients reaching phosphate treatment targets increased from 64% to 74% in HDF patients and was stable in HD patients (66% and 66%); the difference between groups reached statistical significance (P = 0.04). Nutritional parameters and residual renal function were similar in both treatment groups. LIMITATIONS: Only predialysis serum phosphate levels were measured; phosphate clearance could therefore not be calculated. CONCLUSION: HDF may help improve phosphate control. Whether this contributes to improved clinical outcome remains to be established.