Good performance of highflux dialysers means high clearances of molecules up to 15 kD at low ultrafiltration levels, keeping backfiltration as low as possible. On the base of the model of Villarroel [Trans. Am. Soc. Art. Int. Org. (1977)], further developed by Zydney [Artif. Org. (1993) 919] and Legallais [J. Membr. Sci. 168 (2000) 3], we created a tool, which can easily be used with the standard software Mathcad. The membranes and modules, we investigated, were produced by Gambro. Intrinsic properties of membranes for characteristic molecules have been measured by the manufacturer in hollow fiber mini modules. The fluid system, used in our experiments, was an aqueous dextrane, cytochrome C, creatinine solution on blood side and deionized water on dialysate side. In contrast to other authors, we propose to fit Sherwood parameters on dialysate side, since mass transfer of solutes into dialysate is strongly dependent on the geometry (manufacturing, design) of the module. We found an excellent correlation with experimental data and did some interpretation about influences of various parameters (membrane, flow conditions, geometry, etc.) on dialyser performance (clearance, backfiltration).