Patients undergoing hemodialysis manifest marked temporary neutropenia occurring 2 to 15 minutes after the start of dialysis, followed by an increase in band neutrophils. Total leukocyte counts return to near normal levels approximately one hour later. The phenomenon is not due to simple filtration of leukocytes by the dialyzer nor to heparin sodium administration, and is independent of the administration set or the infusion pump mechanism. Rebound leukocytosis is not observed, and the neutropenia is unaccompanied by chills, fever, or other symptoms usually associated with intravenous infusions of pyrogenic substances.