The dynamics of circulating platelet aggregates (CPA) in acute myocardial infarction (MI) was correlated to its extent, localization and clinical outcome. Creatine kinase (CK) and CPA were measured in 30 patients with acute MI 24 hours after its onset, and on the third, fifth and eighth day following the accident. Twelve patients had anteroseptal MI, another 12 had inferior, and the remaining 6 had non-Q wave MI. 24 hours after the accident CPA values differed significantly between the three groups (p < 0.05). The values of CPA increased with increasing CK. In all the patients CPA and CK returned to normal or almost normal values on days 8 and 5 following MI, respectively. Eleven patients (37%) had heart failure: 8 of them (73%) had anteroseptal MI and 3 had inferior MI (27%). In 10 patients with heart failure (91%) and only in 2 out of 19 patients without heart failure (10.5%), CPA peaked on days 3 or 5 after MI (p < 0.05). In all other patients CPA declined steadily from the initial value. These results suggest that platelet aggregability is significantly associated with the severity of MI and with heart failure.