The Beta-Blocker Heart Attack Trial was a randomized clinical trial of propranolol versus placebo in 3837 patients after myocardial infarction. A 24 hour ambulatory ECG was obtained before therapy in 3290 patients 2 to 21 days after myocardial infarction. Sensitivity, specificity, positive and negative predictive values, and prevalence were calculated for four definitions of ventricular arrhythmia with either total or sudden death (death in less than 1 hour of observed symptoms) as an endpoint. These indexes were obtained using the first 1, 2, 4, 6, 12, and 24 hours plus a random hour, a random daytime hour, and a random nighttime hour of the 24-hour ECG of 1336 placebo patients. For both total death and sudden death, as the duration of monitoring increased, (1) prevalence increased, (2) sensitivity increased, (3) specificity decreased, (4) positive predictive value either changed very little or decreased, and (5) negative predictive value was high (greater than 90%) and increased slightly. None of the 3 random hours offered anything beyond the first hour. The Beta-Blocker Heart Attack Trial data, which were based on an average follow-up of 25 months, show that as the number of hours of ambulatory monitoring increase, the percentages of patients identified at risk or not at risk (the positive and negative predictive values) do not change much. Twenty-four hours of monitoring does not appear to be the optimal time duration for deciding whether to treat arrhythmias in patients after infarction.