Most synthetic hemodialysis membranes have an asymmetric structure consisting of a thin semi-permeable skin and a more open support structure. The impact of this asymmetric structure on the transport characteristics of hemodialysis membranes has not been appreciated. Experimental data were obtained for solute clearance and sieving through asymmetric polysulfone membranes and homogeneous cellulose triacetate hollow fiber membranes. The clearance of urea. and the large molecular weight dextrans were greater for the cellulose triacetate membrane compared to the polysulfone, but the clearance of the smaller molecular weight dextrans was slightly greater for the polysulfone dialyzer. The sieving coefficients for the asymmetric membrane were uniformly smaller than those for the homogeneous membrane, although this effect was dramatically reduced at very small filtration velocities. These results were in good agreement with theoretical calculations based on a two-layer model for the asymmetric membrane and available hydrodynamic descriptions of hindered solute transport. (C) 2003 Elsevier B.V. All rights reserved.