Biocompatibility of a dialyzer membrane has been defined largely by the degree to which it activates complement. Modifications of the cellulose membrane and the development of synthetic membranes have minimized the activation of complement and its associated complications. However, less is known about the blood-dialyzer membrane interactions that may occur in membranes made of the same synthetic polymer. A patient is described who developed dialysis-associated thrombocytopenia using a Fresenius Medical Care Optiflux polysulfone membrane (F-160) that significantly improved when switched to the polysulfone Asahi REXEED 25S membrane (AR-25S). A comparison of postdialysis d-dimer level suggests that the F-160 membrane activated the coagulation pathway to a greater extent than the AR-25S. Subtle differences between the internal surfaces of the membranes that are manufacturer specific may be responsible for exposing this patient's unique predisposition to thrombosis and thrombocytopenia. Despite the advances in membrane biocompatibility, differences may exist among membranes made of the same synthetic polymer. Published by Elsevier Inc.