Four types of adverse events resembling immunological reactions have been reported in patients undergoing chronic maintenance hemodialysis: anaphylaxis, hypereosinophilia, asthmatic attacks, and pulmonary leukostasis with compromised pulmonary function. We evaluated three successive chronic dialysis patients who had developed acute hypersensitivity reactions manifested by increased airway obstruction in response to hemodialysis. Each patient also had a non-acute reaction to dialysis, hypereosinophilia, and one patient had urticaria. All dialyzers were ethylene oxide sterilized and none were reused. In these three patients the adverse reactions abated after changing from dialyzers containing Cuprophan to dialyzers not containing Cuprophan. In each patient a second exposure to Cuprophan caused recurrence of one or more of the adverse reactions. These case studies and our re-examination of previous observations suggest that in some patients both acute and chronic adverse reactions may be related to the use of Cuprophan.