The Cardiac Arrhythmia Pilot Study (CAPS) was a 1 year trial that analyzed the safety and effectiveness of arrhythmia suppression in 502 patients surviving acute myocardial infarction who had greater than or equal to 10 ventricular premature depolarizations/h or greater than or equal to 5 runs of ventricular tachycardia on a Holter recording obtained 6 to 60 days after the acute infarction. Because 100 of these patients received placebo in a double-blind fashion for 1 year, a comprehensive objective analysis was performed of spontaneous arrhythmia changes based on real data rather than statistical estimates. In the CAPS placebo group, 19% developed some serious clinical event in 1 year (death, heart failure, proarrhythmia) that could likely be attributable to antiarrhythmic drug toxicity. A significant reduction in the frequency of ventricular premature depolarizations (p = 0.004) occurred in the first few weeks of "therapy" with a further significant (p less than 0.04) decrease between 3 to 12 months. After initiation of placebo antiarrhythmic therapy, 27% had "apparent ventricular premature depolarization suppression" (greater than or equal to 70% reduction) after one Holter recording evaluation and nearly half (48%) after six Holter recordings to assess suppression were performed.(ABSTRACT TRUNCATED AT 250 WORDS)