In 585 patients with a first acute myocardial infarction the maximum activity of heat stable lactate dehydrogenase (EC 220.127.116.11) was correlated with mortality at three months. The patients participated in a double blind trial with metoprolol in acute myocardial infarction. In all patients not taking a beta blocker a highly significant correlation was found, but this was not evident in those who were. When patients with anterior or inferior infarctions treated with a placebo were analysed separately the correlation remained, as it did when the patients who were alive on the fourth day after the onset of pain were analysed separately. No correlation was observed between enzyme activity and three month mortality in these subgroups if only patients treated with metoprolol were included. In a subsample of only 171 patients it was found that the maximum activity of creatine kinase (EC 18.104.22.168) and creatine kinase subunit B did not correlate with three month mortality regardless of treatment. Thus it is concluded that when a sufficiently large number of patients are investigated there is a highly significant correlation between the enzymatic estimation of infarct size and early mortality in acute myocardial infarction. This relation did not persist when patients treated with beta blockade were analysed.