Background and Aims Short daily hemodialysis (SDHD) has improved outcomes observed in conventional thrice-weekly hemodialysis (CHD). Analogously, short daily hemodiafiltration (SDHDF) has also improved outcomes observed in conventional thrice-weekly hemodiafiltration (CHDF). Furthermore, while high-volume CHDF has been proclaimed to be superior to CHD, there have been no studies comparing SDHDF to SDHD. We performed a comparison of clinical, laboratory, echocardiographic and quality of life features in dialysis patients treated by SDHD or SDHDF. Method Twelve patients (mean age 60.8±15.4 years; 7 males; AVF 7, AVG 1, Catheter 4) on regular in-center SDHD program were studied in a longitudinal, prospective, non-randomized, single-subject A-B-A design comparing high-flux SDHD to post-dilution on-line SDHDF. Patients had been for at least 6 months on SDHD (105-150min, 6x/week; SDHD1) and were clinically stable before conversion to SDHDF (105-150min, 6x/week, mean total convective volume of 63.57±5.44 L per week) for 6 months. Following that, all patients were switched back to SDHD for another 6 months (SDHD2). Data were collected at the end of each period. Dialysis parameters throughout the study were matched and set as follows: high-flux polysulfone dialyzers (BBraun Xevonta Hi 23®), blood flow 300–350 mL/min, dialysate flow 500 mL/min and ultra-purified water (Aquaboss heat disinfection osmosis). The results were expressed as mean ± standard deviation. Each patient served as his/her own control. Repeated measures analysis of variance (ANOVA) were used. Results Slightly higher predialysis mean arterial pressure (MAP) levels were observed during SDHDF (102.51±11.28 mmHg) compared to SDHD1 (MAP 95.5±9.82 mmHg, p<0.01) or to SDHD2 (97.61±11.19 mmHg, p=0.03), without significant differences in intradialytic blood pressure variability. Similarly, there was an increase in predialysis hemoglobin levels (Hb) during SDHDF (Hb 12.39±1.2 g/dL), compared to SDHD1 (Hb 11.30±1.09 g/dL, p=0.02) or to SDHD2 (Hb 11.23±1.84 g/dL, p<0.01), although there were no significant differences in erythropoietin-stimulating agent or iron supplementation weekly doses, transferrin saturation rates or ferritin levels. Solute kinetics measurements showed higher myoglobin clearance during SDHDF (45.95±11.0 mL/min) compared to SDHD1 (20.11±5.2 mL/min, p<0.01) or to SDHD2 (21.39±3.55 mL/min, p<0.01). Likewise, there was a slight increase in β2-microglobulin removal in SDHDF (64.92±10.4 mL/min) compared to SDHD1 (59.78±17.0 mL/min, p<0.01) or to SDHD1 (53.01±7.71 mL/min, p<0.01), whereas there were no significant differences in urea and creatinine clearances. On the other hand, predialysis osteometabolic (serum PTH, calcium, phosphate, alkaline phosphatase, sodium, potassium, glucose and glycated hemoglobin), inflammatory (serum c-reactive protein and interleukin-6) or nutritional (serum albumin, total protein and lipid profile) parameters did not change significantly during the study. Left ventricular mass index, left atrial volume index and global longitudinal strain, assessed by echocardiography at the end of all 3 periods, were similar, as well as brain natriuretic peptide and homocysteine serum levels. Additionally, there were no significant changes in any quality of life features evaluated by Kidney Disease Quality of Life-Short Form, or symptoms assessed by Edmonton Symptom Assessment System Revised Renal, throughout the full course of the study. Conclusion In spite of mild increase of predialysis mean arterial pressure and hemoglobin levels along with greater removal of middle molecular weight solutes, no others clinical, laboratory, echocardiographic or quality of life findings differed significantly during the three periods. Together, these data do not provide evidence of clinically meaningful benefit from on-line hemodiafiltration over high-flux hemodialysis, both performed six times a week.