K-induced cardioplegia during anoxic arrest was utilized in a study of 190 consecutive patients undergoing revascularization (average 2.8 grafts/patient). Surgical technique, moderate systemic hypothermia with intermittent anoxic arrest and the surgeon were the same for all patients. Patients (135), KCl-treated, received a bolus (150 ml) of K solution injected into the proximal aortic root whenever the aortic cross-clamp was applied; 55 others served as control subjects. The mortality rate was 2.2% (3 of 135) in the KCl-treated group and 1 of 55 in the control group. New Q waves appeared in 5.9% (8 of 135) of the KCl-treated patients and 11% (6 of 55) of control subjects (P = NS [not significant]). Catecholamine drips were required after bypass in 4.4% (6 of 135) of patients given K and 18% (10 of 55) of control patients (P < 0.05). Profound myocardial relaxation was of added technical value with K. Hearts treated with K exhibited more prompt cardioversion, separated from cardiopulmonary bypass with less need for inotropic support and exhibited less myocardial injury during the revascularization procedure.