The morphologic changes in the myocardium of hypocontractile segments, especially the possible structural counterparts underlying chronic ischemia, are not well documented. Light and electron microscopy was performed on myocardium derived from the anterior wall of the left ventricle of 98 patients during coronary artery bypass grafting. Wall motion data were collected from the region corresponding to the biopsied zone. The changes seen in a substantial part of the cardiomyocytes corresponded to ''dedifferentiation'' rather than degeneration characteristics. The affected cardiomyocytes showed a partial to complete loss of sarcomeres, sarcoplasmic reticulum, and T-tubules and presented abundant plaques of glycogen, strands of rough endoplasmic reticulum, lots of minimitochondria, and a tortuous nucleus. The volume of the cells was similar to that of normal cells. The number of the affected cells was consistently higher in endocardial parts than in epicardial ones. The cell changes occurred in the myocardium of patients both with and without a previous Q-wave infarction. There was a significant relation between anterior wall motion abnormalities and the incidence of affected cells in the endocardium, but not in the epicardium. A significant relationship was found in noninfarcted patients between the presence of affected cells and the amount of connective tissue. Furthermore, in these patients the number of affected cells depended on the degree of stenosis. It is proposed that segments in which these structural changes prevail will not recover immediately after revascularization but that they might show a delayed recovery of function, because structural remodeling requires time in order to regain sufficient contractile material.