The effectiveness of deep systemic hypothermia (20 degrees C) in myocardial protection during aortic cross-clamping was elevated. Seventy-one consecutive patients undergoing coronary artery bypass grafting were divided into two groups. In group A (32 patients) systemic temperature was reduced to an average of 26.8 degrees C (range 24 degrees to 28 degrees C) and the amount of cardioplegic solution infused totalled 1,000 to 1,200 cc. In Group B (39 patients) systemic temperature was reduced to an average of 20.8 degrees C (17 degrees to 23 degrees C) and the total amount of cardioplegic solution infused was 100 to 300 cc. The mean number of coronary (distal) anastomoses per patient was 4.46 in Group A and 4.51 in Group B. There were no surgical deaths, perioperative infarcts, or neurologic damage in either group. Postoperative catecholamine dependence was used as an indicator for inadequate myocardial protection. Catecholamine support was required by 18 patients (56.25%) in Group A and two patients (5.13%) in Group B (p less than 0.0001). Patients of both groups who received five or six coronary anastomoses, whose aortic cross-clamp time was 60 minutes or more, and whose preoperative left ventricular ejection fractions were above 50% were compared: Ten of the 11 (91%) in Group A required catecholamine support as opposed to none of the 12 in Group B (p less than 0.0001). No significant difference in the incidence of catecholamine requirement was found between patients of both groups whose aortic cross-clamp time was less than 60 minutes (2/13 patients in Group A and 2/21 patients in Group B), regardless of their preoperative left ventricular ejection fraction. We conclude that when aortic cross-clamp time exceeds 60 minutes, that is, when multiple distal anastomoses are performed, deep systemic hypothermia is a simple and effective method for myocardial preservation.