Fifty-five patients were hospitalized in a Coronary Care Unit within the first 24 hours after onset of symptoms of an acute myocardial infarction. The sum of positive and negative ST-segment deflections of their twelve leads electrocardiograms (epsilon ST12) showed a maximum within the 3rd hour from the onset of symptoms. Thereafter, in the first 24 hours, there was a marked reduction in epsilon ST12, with a steep and significant fall within the 7th hour from symptoms. In the following nine days of this study, the patients showed 4 different epsilon ST trends, but there was no significant correlation with CPK curves, or with the kind of therapy the patients underwent. An irregular trend of epsilon ST12 or a secondary late rise (after 36 hours from symptoms) suggest an unfavorable prognosis (1 death in the epsilon ST-3 group, and 2 deaths + 1 ventricular fibrillation in the epsilon ST-4 group). No significant difference results between the patients treated with high doses Heparin plus Acetyl-Salicylic-Acid (A.S.A.), and the patients treated with A.S.A alone, though the first treatment seems to reduce the values of epsilon ST12 more rapidly; perhaps this behaviour is due to the use of antiplatelet drug A.S.A in both groups, and to the relatively small number of patients.