To determine a possible mechanism for the previously observed improved outcome after bypass surgery in patients with poor ventricular function and viable myocardium, we sought to examine the relation between the extent of viability and the frequency of an abnormal signal-averaged electrocardiogram (SAECG) in patients with reduced ejection fraction and coronary artery disease. Fifty-two patients with coronary disease and ventricular dysfunction underwent quantitative redistribution thallium-201 (Tl-201) scintigraphy at rest to determine the extent of viability. The presence of late potentials was assessed by SAECG. Long-term, cardiac event-free survival was determined. Patients with greater viability (group 1, n = 23) were similar to patients with less viability (group 2, n = 29) with respect to age, gender, ejection fraction, and incidence of arrhythmia. Fewer group 1 patients had late potentials (33% vs. 65%, p = 0.05) and individual parameters were significantly more abnormal in the group 2 patients. Patients with late potentials had less viability than patients without late potentials (viability index 0.61+/-0.15 vs. 0.69+/-0.14, respectively, p = 0.05). By multivariate analysis, only the extent of viable myocardium and the left ventricular end-diastolic pressure were independent predictors of late potentials. Survival free of cardiac death or transplantation was better in patients with a normal SAECG (p<0.04) and in patients with predominantly viable myocardium (p<0.005). Thus, patients with low ejection fraction, coronary disease, and viable myocardium have a lower frequency of late potentials, suggesting reduced susceptibility to ventricular arrhythmia.