A 61-year-old woman suffered an inferior wall myocardial infarction on the third day following aortocoronary bypass surgery. Her condition subsequently deteriorated and she was not responsive to vigorous conventional therapy. On the twenty-first postoperative day, selective coronary arteriography revealed a patent circumflex bypass graft and an occluded right coronary artery bypass graft. Left ventricular injection showed slight dilation of the ventricle and a reduced ejection fraction. A dissection from the left ventricular cavity into the myocardium, across the septum near the apex of the left ventricle with rupture into the right ventricle, was seen functioning as a left-to-right shunt with a step-up in oxygen saturation at that level. This was corrected by operation on the twenty-ninth postoperative day and, 9 months after operation, the patient remains asymptomatic and has normal exercise tolerance.