Much progress has been made in the biocompatibility of hemodialysis membranes with the development of synthetic membranes. However, despite better biocompatibility, thrombocytopenia has been observed in patients who are dialyzed with various polysulfone membranes. Recent publications have attributed this phenomenon to the electron-beam sterilization of dialyzers. We report 5 cases of thrombocytopenia in patients treated by hemodialysis with an electron-beam sterilized polysulfone membrane. The thrombocytopenia resolved after switching to either of 2 polysulfone membranes, one of which also was electron-beam sterilized. Although we were unable to elucidate the exact biochemical mechanism, the cause of the thrombocytopenia in this series appears to be related to the type of polysulfone and not due to the sterilization technique.