Abstract Background and Aims Ultrafiltration coefficient (KUF) is a measure of dialyzer permeability to water, given by manufacturers, and expressed in milliliters/hour/mmHg of pressure gradient. Most dialyzers used at this moment are classified as high-flow as having a KUF >20 mL/h/mmHg. KUF value of a dialyzer is an “in vitro” measure which does not correlate with “in vivo” measure, stated to be 5-30% less than theoretical KUF. According to this statement, we wanted to study differences between theoretical and real KUF of two dialyzers with the same membrane composition and different surface area. For this task, we chose 2 dialyzers of usual use in our dialysis unit: FX CorDiax 1000 and FX CorDiax 80 (®FMC). These dialyzers have a main difference in Surface area: 2.3 and 1.8 m2, respectively, and have a theoretical KUF of 76 and 64 mL/h/mmHg. Also, both dialyzers differ in intern diameter (210 vs 185 µm). Method A prospective, not randomized trial was conducted. Inclusion criteria were patients in chronic dialysis at our center, in postdilutional online hemodiafiltration with a FX1000 dialyzer. All patients were informed and accepted freely signing an informed consentment. On 2 consecutive weeks, the intermediate dialysis session of the week was performed in a DBB-EXA dialysis monitor (®Nikkiso), first with a FX CorDiax 1000 and next week with a FX CorDiax 80, with habitual patient dialysis parameters (240 minutes). Blood tests were taken previous and after the session to evaluate clearance differences between both filters. During the session, pressure and flow data was recorded, with an “in vivo” KUF measurement given by the monitor. Results Dialysis sessions were performed in 12 patients who met inclusion criteria. Data results are shown in Table 1. Significant differences between FX CorDiax 1000 and FX CorDiax 80 were observed in prefilter pressure (p 0.008), convective volume (p 0.034), prolactin reduction ratio (p 0.012), “in vivo” KUF/ “in vitro” KUF ratio (p 0.002). “In vivo” KUF graphic given by the monitor showed a logarithmic function curve, with a stabilization between the 2nd and 3rd hour of the session. Conclusion In our study, measurement of “in vivo” KUF by Nikkiso monitor showed a slow decrease after 2nd hour of session, probably related with protein cake of membrane pores. “In vivo” KUF measure was approximately a 40-45% of theoretical value given by manufacturer. The “in vivo” KUF/ “in vitro” KUF ratio was higher for the smaller dialyzer, which probably means a higher optimization of its surface area with usual dialysis sessions. Convective volume and prolactin reduction ratio, as parameters of dialysis efficacy, are higher in the dialyzer with higher surface area and inner intern diameter.