IgA anaphylactic transfusion reactions are rare events, estimated to occur in 1 in 20,000 to 47,000 transfusions. The signs and symptoms of these reactions do not differentiate them from other causes of anaphylaxis. The diagnosis of an anaphylactic transfusion reaction is established by showing an IgA-antibody in the patient's serum. Most laboratories that test for IgA antibodies rely on the PHA method, which uses red blood cells that are coated with serologically defined IgA multiple myeloma proteins. We tested sera referred from Red Cross regional blood centers and hospitals from patients with suspected IgA anaphylactic reactions and found an IgA antibody in 76.3% of IgA-deficient patients. However, only 17.5% of all samples referred contained an IgA antibody, indicating that most persons with suspected IgA anaphylactic reactions had experienced acute generalized reactions that were from causes other than anti-IgA transfusion. Using PHIA to measure serum concentrations of IgA and PHA to detect IgA antibodies, we found the frequency of IgA deficiency (< 0.05 mg/dL) and class-specific anti-IgA in random blood donors to be approximately 1 in 1,200. Titers of anti-IgA did not distinguish these seemingly healthy blood donors from patients with a history of an anaphylactic transfusion reaction. Because the frequency of 1 in 1,200 greatly exceeds the observed frequency of anaphylactic reactions in transfused persons, we conclude that using PHA for anti-IgA does not reliably predict risk for an anaphylactic transfusion reaction. Additional research is needed to define a more specific marker to identify those persons who are truly at risk for these serious, but rare, complications of blood transfusion.