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. Short-term analysis of a secondary outcome of an ongoing randomized controlled trial. 493 (84%) consecutive patients from 589 patients included in the Convective Transport Study (CONTRAST) by January 2009 from 26 centers in 3 countries. Online hemodiafiltration (HDF) versus continuation of low-flux HD. 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). Phosphate, use of phosphate-binding agents, and proportion of patients achieving treatment targets at baseline, 3 months, and 6 months. 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. Only predialysis serum phosphate levels were measured; phosphate clearance could therefore not be calculated. HDF may help improve phosphate control. Whether this contributes to improved clinical outcome remains to be established. Copyright 2009 National Kidney Foundation, Inc. Published by Elsevier Inc. All rights reserved.