Levels of serum cortisol, plasma free fatty acids (FFA), and urinary catecholamines were collected in 31 patients with acute myocardial infarction on the day of admission to the coronary care unit (samples obtained from 15 patients with diseases other than myocardial infarction were considered as controls). These values were correlated with the presence or subsequent development of left ventricular failure, arrhythmias, shock, or death. Sixteen of 17 infarction patients without the above complications had cortisol levels less than 20 μg%; 10 of 12 patients with complications had higher cortisol levels. All of the infarction patients without complications had plasma FFA levels less than 1100 μEq/liter, while six of nine patients with complications had levels of 1100 μEq/liter or higher. Fourteen of 16 patients without complications had urinary catecholamines less than 12.5 μg%, while seven of 10 patients with complications had levels greater than this. Of interest were a few patients without complications on admission, but with elevated cortisol or FFA levels, who developed complications on the following day. The results of the present study suggest that the generalized metabolic stress of acute myocardial infarction results in elevations of cortisol, FFA, and catecholamines as measurable biochemical indicators and/or predictors of the severity of the infarction.