Coronary bypass surgery may be associated with an increased perioperative mortality rate in patients with unstable compared to stable angina. The mortality rate is excessively high when surgery is performed during evolving myocardial infarction. Elevated plasma MB CPK isoenzyme activity is a remarkably sensitive and specific marker of myocardial damage. Accordingly, the authors studied 111 patients with unstable angina to determine whether exclusion of patients with initially elevated MB CPK improves the perioperative mortality rate. Plasma MB CPK activity was assayed prior to catheterization and every 2 hours thereafter. Of the 111 patients, 16, with initially elevated MB CPK activity, were excluded and managed medically. Catheterization was performed in 59 patients, and severe vessel obstruction was documented in 55. Coronary bypass surgery performed in 47 patients was associated with a mortality rate of 4 per cent. Thus, after exclusion of patients with evolving infarction by MB CPK isoenzyme analysis, catheterization and coronary bypass surgery in patients with unstable angina resulted in a mortality rate comparable to that in patients with stable angina.