The relative prognostic significance of ventricular tachycardia and ventricular fibrillation inducible at programmed stimulation within 1 month of acute myocardial infarction was compared in a prospective study of 403 clinically well survivors of transmural infarction who were 65 years old or younger. The prognostic significance of delayed potentials on the signal-averaged electrocardiogram was also examined in a subset of 306 patients without bundle branch block. Among the study patients, 20% had inducible ventricular tachycardia, 14% had inducible ventricular fibrillation, and 66% had no inducible arrhythmias. The 2 year probability of remaining free from cardiac death or nonfatal ventricular tachycardia or fibrillation was 0.73 for those with inducible ventricular tachycardia, 0.93 for those with inducible ventricular fibrillation, and 0.92 for those with no inducible arrhythmias. The cycle length of inducible ventricular tachycardia was 230 msec or more in 70% of the patients with inducible tachycardia who died. Of the patients studied by signal-averaged electrocardiography, 26% had delayed potentials. At 2 years, the probability of remaining free from cardiac death or nonfatal ventricular tachycardia or fibrillation was 0.73 for patients with delayed potentials and 0.95 for patients with no delayed potentials. There was a significant correlation (p < .001) between the presence of delayed potentials and the ability to induce ventricular tachycardia. In conclusion, in survivors of recent infarction who have not had spontaneous ventricular tachycardia or fibrillation, inducible tachycardia (but not inducible fibrillation) at programmed stimulation predicts a significant risk of death or spontaneous tachycardia or fibrillation. A similar risk is found for patients with delayed potentials on the signal-averaged electrocardiogram.