The a priori hypothesis that diltiazem would reduce the frequency and repetitiveness off ventricular arrhythmias was tested 3 months after myocardial infarction in patients participating in the Multi-center Diltiazem Postinfarction Trial. After 3 months of follow-up, 1,546 of the 2,466 patients enrolled had a 24-hour continuous electrocardiographic recording that contained ≥ 12 hours of analyzable data. They were similar to the patients who survived 3 months but chose not to have a 24-hour ehctrocardiographic recording (i.e., they were representative of the entire group that survived 3 months). After 3 months of follow-up, there were no significant differences between the diltiazem and placebo groups in the prevalence of atrioventricular block, the frequency of atrial arrhythmias or the frequency or repetitiveness of ventricular arrhythmias. Heart rate was significantly lower (67 ± 12 vs 71 ± 12 beats/min) and there was a significantly greater proportion of patients with sinus pauses ≥ 2 seconds in duration in the diltiazem group (6%) than in the placebo group (3%). Comparison with placebo revealed no evidence either for an anti- or proarrhythmic effect of diltiazem. There was no reduction in sudden or arrhythmic death attributable to diltiazem treatment; the fraction of total deaths that were arrhythmic by the Hinkle classification was 41% in the placebo group and 42% in the diltiazent group. It may be that the lack of effect of diltiazem on ventricular arrhythmias is partially responsible for its lack off effect on mortality after myocardial infarction.