While studying the immunological response to acute myocardial infarction (AMI) we noticed that patients who on admission had a high serum immunoglobulin E (IgE) level were less likely to die suddenly. This observation seemed to deserve verification since atopic patients whose production of IgE was excessive had been reported to have depressed haemostatic platelet function and impaired thrombinogenesis. We measured levels of serum IgE in 386 patients with AMI at the time of admission to the coronary care unit. Patients were divided into two groups, depending on the presence (n = 55) or absence (n = 331) of sudden cardiac arrest. The two groups did not differ with respect to age, sex, or risk factors for coronary artery disease. The mean level of serum IgE was significantly higher in the group without sudden cardiac arrest than in the group with this complication. In a separate study we found that high serum IgE levels were associated with delayed thrombin generation in the clotting blood of survivors of myocardial infarction. Early determination of serum IgE levels might help to detect patients at risk of sudden cardiac arrest during myocardial infarction. Patients with high serum IgE levels might be protected against sudden cardiac death through the depression of clot formation because of the late appearance of thrombin in their coronary arteries.