We suggest that many severe hypersensitivity reactions during dialysis are IgE mediated (classical anaphylaxis). Besides ETO there must be other substances involved which are not extractable from the dialyzer (e.g. heparin). Some severe reactions are not explained by high IgE levels in the serum. However, in vitro complement activation gives no indication of an anaphylatoxin mediated reaction mechanism, at least in the severe cases of hypersensitivity.