It has been proposed that a single preoperative dose of a corticosteroid may protect the myocardium from ischemic injury during open heart surgery. To test this hypothesis, a prospective, randomized, double blind study was carried out in ninety-five patients undergoing coronary bypass surgery using intermittent ischemic arrest with systemic and local hypothermia. Half the patients received 2 gm (approximately 30 mg/kg) of methylprednisolone 2 hours prior to the initiation of cardiopulmonary bypass and the other half received a placebo. Postoperative electrocardiograms and blood levels of serum creatine phosphokinase (CPK), lactic dehydrogenase (LDH), and serum glutamic oxalacetic transaminase (SGOT) were compared in the two groups. No apparent difference was noted in the number of patients with significantly elevated levels of CPK, LDH, or SGOT or in the number with positive isoenzyme patterns of CPK and LDH. Moreover, there was no significant difference in the mean values of CPK, LDH, or SGOT between the two groups. The number of patients with electrocardiographic evidence of myocardial injury (10 per cent) was the same in both groups and no difference was noted in (1) the ease with which patients could be weaned from cardiopulmonary bypass, (2) postoperative arrhythmias, (3) postoperative bleeding, (4) postoperative respiratory insufficiency, and (5) length of hospital stay. It is concluded that a single preoperative dose of 2 gm of methylprednisolone offers no demonstrable protection to the myocardium from the effects of ischemia during coronary artery bypass surgery.