Holter monitoring of 51 patients on the 4th to 6th day after acute myocardial infarction (AMI) disclosed complex ventricular ectopic beats in 17 (35%), nine of whom (53%) died in the following 26 months. Of the remaining patients with no or only non-complex ectopic beats, five (15%) died. The difference was significant using Gehan's test of censored data (p less than 0.01). Multivariate analysis demonstrated a significant increase of death risk associated with complex ventricular ectopic beats as well as with heart failure (relative risk 3.6 and 5.9, respectively). Age, gender, number of ventricular ectopic beats, cardiac enlargement on X-ray and angina pectoris had no significant effect on mortality. The prevalence of ventricular ectopy was 84% (n = 43) irrespective of age and sex and equal to the finding in the late hospital phase reported by other investigators, and in healthy subjects. Complex ventricular ectopic beats appeared in 35% (n = 17). The prevalence increased significantly with age and was significantly higher in post-infarction patients (38%) than in healthy individuals (8%). The present results indicate that a subgroup containing most of the high-risk patients may be identified early after AMI by detecting complex premature ventricular beats in continuous electrocardiographic recordings.