Internal filtration/backfiltration (IF/BF) of a dialyzer depends on several parameters. This study evaluated the effect of the blood flow rate (Q (B)) on the internal filtration flow rate (Q (IF)) measured using Doppler ultrasonography for a high-flux dialyzer with a polysulfone membrane, APS-15E. In an in vitro study, bovine blood was circulated through the dialyzer, at a Q (B) of 100-350 mL/min. The clearances (CL) of creatinine, β(2)-microglobulin, and α(1)-microglobulin were then investigated. Q (IF) increased with the Q (B) value. A good correlation was obtained between Q (IF) and the pressure difference between the pressures at the inlet of the blood compartment and the pressure at the outlet of the dialysate compartment. The creatinine CL values strongly depended on Q (B) because molecular diffusion was dominant. The β(2)-microglobulin CL also depended on Q (B), because its removal rate seemed to be affected by both diffusive and convective transport caused by the IF/BF. An extremely low CL value was obtained for α(1)-microglobulin because of its low diffusivity and membrane fouling induced by proteins plugging the membrane. In conclusion, the IF/BF in the dialyzer strongly depends on Q (B). Furthermore, the dependence of the solute clearance on Q (B) decreased with increasing molecular size of the solute because of the decrease in diffusivity through the membrane.