Myocardial tissue taken from 19 autopsy cases of myocardial infarction were examined both by the nick and labeling method (NELM) and by DNA agarose gel electrophoresis in order to demonstrate the localization of cells with fragmented DNA and to confirm the internucleosomal cleavage of DNA biochemically. The nuclei corresponding to those with the histological features of acute myocardial infarction in hematoxylin and eosin (H&E)-stained sections were stained strongly positive with the nick end labeling method. Myocardial cells corresponding to those with nick end labeling method-stained nuclei, on the other hand, had mostly pyknotic and karyolytic nuclei and some unremarkable nuclei, even nuclear ghosts, and showed degenerated cytoplasm, including contraction band necrosis in H&E-stained preparations. The agarose gel electrophoresis of DNA extracted from the corresponding areas mentioned above showed the ladder pattern of internucleosomal cleavage characteristic of apoptosis. The present study revealed that infarcted myocardial cells with nuclear outlines, even nuclear ghosts, showed a distinct DNA fragmentation with the ladder pattern of internucleosomal cleavage. It is concluded from this study that the damaged myocardial cells of acute myocardial infarction represent a coagulation necrosis having the biochemical nature of apoptosis.