Solute removal performances of dialyzers are dependent not only on the solute permeabilities of the membrane but also on the module design. We have investigated how the packing density of hollow fiber (PDF) affects the solute removal performances. A series of 4 polyester polymer alloy membrane test dialyzers were assembled with varying PDFs of 29.6%, 35.3%, 44.1%, and 53.1%. Clearances (C(L)) were measured in vitro for creatinine (MW113), vitamin B(12) (MW1355), and chymotrypsin (MW25300) with various Q(B)=100 to 400 and Q(D)=350 to 650 mL/min in the absence of net ultrafiltration. When Q(B) was 44.1%, which was also considered to be due to the internal filtration. Packing density of hollow fiber can be optimized in terms of solute removal performances when the target solute and therapeutic conditions are specified.