Using thallium myocardial scintigraphy and radionuclide ventriculography, we assessed size of infarction and left ventricular function at late follow-up (greater than 2 months) in 13 patients who underwent emergency coronary artery bypass surgery (ECABS) during evolving myocardial infarction and in 26 controls who received conventional treatment for acute infarction. Thallium scans were quantitatively analyzed. The thallium defect, expressed as a numerical value in arbitrary units, was smaller after early revascularization (within 4 hr of the onset of symptoms, n = 10) than in the controls: 397 +/- 232 vs 2779 +/- 972 for anterior infarction (p less than .001) and 475 +/- 511 vs 1454 +/- 960 for inferior infarction (p less than .05). The patients undergoing revascularization late (4 to 5 hr after the onset of symptoms, n = 3) had thallium defects comparable to those in the controls. Regional ejection fraction of the involved left ventricular segment was higher after early revascularization (41 +/- 9% vs 21 +/- 8% for anterior infarction, p less than .005; 67 +/- 14% vs 51 +/- 11% for inferior infarction, p less than .01). Global ejection fraction was higher after early revascularization in patients with anterior infarction (57 +/- 10% vs 37 +/- 9%, p less than 0.02), but not in those with inferior infarction (60 +/- 11% vs 54 +/- 8%, p greater than .05). After late revascularization, regional and global ejection fraction were comparable to those in controls. In selected patients, early reperfusion of acutely ischemic myocardium by ECABS can limit size of infarction and preserve left ventricular function, but time constraints may be severe.