Intraoperative myocardial infarction is a recognized complication of aortocoronary bypass surgery. One major cause of such infarction may be interruption of coronary blood flow, particularly in patient with poor coronary collateral circulation. In 30 patients use of an intracoronary shunt made it possible to limit the period of coronary occulusion during graft construction to a few minutes. Use of this shunt was associated with a reduced incidence of intraoperative myocardial infarction (as judged by the appearance of new Q waves) when these patients were compared with 50 patients operated on without this procedure (6 of 50 [12 percent] versus 0 of 30). The incidence of postoperative persistent S-T segment elevation was reduced from 21 of 50 (42 percent) to 5 of 30 (17 percent). Except for use of the shunt, the surgical technique was identical in the two groups of patients.