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While previous clinical studies have shown a possible beneficial effect of the reperfusion performed at a relatively late phase of acute myocardial infarction (“late reperfusion”) in preventing left ventricular enlargement, the mechanism has not been clarified.Of 89 patients with an initial anterior myocardial infarction, reperfusion was successful in 69. These 69 were divided into three groups according to the time required to achieve reperfusion after the onset of symptoms: early-reperfused (<3 hours from the onset to reperfusion; n=22), intermediate-reperfused (3 to 6 hours from the onset to reperfusion; n=28), and late-reperfused (>6 hours from the onset to reperfusion; n=19). The 20 patients whose infarct-related artery were occluded in the acute phase as well as 1 month later was classified as nonreperfused. Infarct size, evaluated as defect volume by 'ITl single-photon emission computed tomography 1 month after the onset, was 1593±652 units (mean±SD) in the late-reperfused group, sigificanti larger (P<.05) than that of the intermediatereperfLsed (106:t546 U) or the eariy-reperfused groups (372±453 U) but not different from that of the nonreperfused group (1736±562 U). Wall motion abnormality index as well as global ejection fraction evaluated by left ventriculography 1 month after the onset showed that late reperfusion did not preserve the left ventricular wall motion and function. These results indicate that the earlier reperfusion decreased the size of the infarction and preserved left ventricular function, whereas late reperfusion (>6 hours after onset) did not limit infarct size or preserve left ventricular function. In contrast, the end-diastolic volume index did not differ significantly among the early-reperfused (50±15 mL/m2), intermediate-reperfused (54±14 mfm2), and late-reperfbsed (53±19 mldm2) groups; those were significantly smaller than that of the nonreperfused group (68±12 mnlm2; P<.05). Left ventriculographic data obtained in both the acute and chronic phase in 39 patients showed that left ventricular volumes increased significantly during the course of myocardial infarction only in the nonreperfused group.Concusions. Late reperfusion appeared to prevent ventricular dilatation following acute myocardial infarction independent of the limitation of infarct size.