Transmural left ventricular biopsies from 13 patients undergoing cardiopulmonary bypass were studied. The hypertrophic myocardium was protected by deep hypothermia (15 degrees C.) during ischemic arrest for a maximal period of 96 minutes. Biopsies were taken at the start of bypass, at the end of aortic cross-clamping, and after 20 minutes of reperfusion. The structure of the mitochondria remained normal, whereas cellular alterations in the form of widened intercalated discs, interstitial and intracellular edema, myelin figures, and slight myofibrillar lysis were observed in all stages. As no clear correlation between these structural injuries and aortic cross-clamping time was seen, they are a result of cardiac hypertrophy rather than intraoperative ischemia.