BACKGROUND: The body of evidence suggests that high volume postdilution online hemodiafiltration (OL-HDF) improves morbidity and mortality in patients with the end-stage renal disease compared to conventional hemodialysis (HD). However, the efficacy of high volume predilution OL-HDF compared to high volume postdilution OL-HDF has rarely been described in observational studies. METHODS: This study was a prospective, single-center, open-label, and controlled randomizedpilot study. Twenty prevalent HD patients on a three-times-per-week schedule from the Tung's Metroharbour Hospital Dialysis program were recruited into the study after obtaining informed consent. Patients were randomized (1:1) to two study groups: the predilution OL-HDF group and the postdilution OL-HDF group. All patients had completed all tests in this study. The overall clearance of middle molecules and protein-bound toxins, as well as pre-to-post reduction ratios (RR) of middle and small molecules, were examined. The measurement was repeated in the second week with the convective volume augmented in both groups. RESULTS: There is no significant difference in the reduction rate and clearance of most studied uremic toxins between the two groups except for fibroblast growth factor-23 (FGF-23) and α1-microglobulin. The RR of FGF-23 and α1-microglobulin were greater in postdilution OL-HDF than in predilution OL-HDF (P = 0.0096 and 0.01). CONCLUSION: Compared to high volume (convection volume, 20 L/session and 23 L/session) postdilution OL-HDF, high volume (convection volume, 48 L/session and 55 L/session) predilution OLHDF is not inferior in the clearance of larger middle molecules and protein-bound uremic toxins and therefore could be a feasible alternative mode of OL-HDF.