Over a 20-month period, we observed 15 anaphylactoid reactions in six patients undergoing hemodialysis and three others on hemofiltration with AN69 capillary dialyzers who were receiving angiotensin-converting enzyme (ACE) inhibitors. These reactions were severe in 11 cases. In eight patients, anaphylactoid reactions stopped when AN69 membrane was replaced by polysulfone membrane and ACE inhibitors were continued. In one case, reducing the dose of ACE inhibitors was sufficient to prevent new reactions. Anaphylactoid reactions did not occur in patients undergoing dialysis with another membrane (cellulosic or synthetic), nor in those on AN69 membrane without ACE inhibitor treatment. We conclude that back-filtration of endotoxin-contaminated dialysate does not play the main role in the origin of such reactions, since they occurred in patients on hemofiltration with sterile and pyrogen-free substitution liquids.