Serum selenium concentration was determined in 49 patients with acute myocardial infarction within 4 hours after the beginning of the symptoms. The mean serum selenium concentration of the patients was significantly lower than that of healthy controls (55 +/- 15 micrograms/l vs. 78 +/- 11 micrograms/l). Among the 49 patients with acute myocardial infarction 20 (41%) had serum selenium concentration below the 95% percentile of the healthy control group. It is concluded that the low serum selenium concentration was present in these patients before the acute event and was not a consequence of the myocardial infarction. No relationship was found in this study between the serum selenium concentration and the severity of myocardial infarction if the number of coronary vessels occluded is taken as the criterion of severity. Serum selenium concentration was similar in patients with 1 or more coronary vessels occluded. Patients with anterior or posterior myocardial infarction had similar serum selenium concentrations. A positive correlation was observed between serum selenium concentration and total serum creatine kinase (CK) activity and serum myoglobin (MB). The serum selenium concentration correlated negatively with the ratio CK-MB/total CK activity, which can be interpreted as minor injury of mitochondria during infarction in patients with normal serum selenium concentration.