Severe reactions during dialysis occurred in 1.7% of hemodialysis patients. Respiratory distress, agitation, pruritus, and alterations in BP were the dominant clinical findings, and one patient suffered a respiratory arrest. It is not clear whether this syndrome is due to toxic substances or to an allergic reaction, although the presence of eosinophilia in three of five affected patients would suggest the latter mechanism. Disconnecting the patient from the extracorporeal circuit invariably improved the symptoms. Current evidence implicates the dialyzer as the most likely culprit, and our experience suggests that none of the commonly used dialysis membranes are truly biocompatible.