Alpha 1-antitrypsin serum levels were measured in 48 patients with acute myocardial infarction and in 19 control patients either with coronary heart disease without necrosis, or with neither coronary disease nor inflammation. Alpha 1-antitrypsin was significantly raised in the group of patients with acute myocardial infarction. As some patients individually showed no change in alpha 1-antitrypsin levels, however, they were divided into two groups according to the maximum serum levels attained. Patients with non-increasing levels of alpha 1-antitrypsin showed increased mortality and a higher incidence of cardiogenic shock, whereas reinfarction occurred more frequently in the group with high alpha 1-antitrypsin levels. Our findings may suggest that the course of a myocardial infarction is determined not only by the severity of the ischaemic event, but also by the response of the "acute" phase reaction" mechanism. We conclude that a failure of alpha 1-antitrypsin levels to increase after myocardial infarction may be associated with a worse clinical course.