Concealed bigeminy represents a rhythm disorder in which bigeminy is ever-present, but the ventricular premature beats are not always detectable and there is an odd number of interectopic sinus beats. Retrospective examination of the ECG tracings of patients with acute myocardial infarction and ventricular premature beats demonstrated a statistically significant predominance of odd numbers of interectopic sinus beats. There were 20 cases of concealed bigeminy and in 10 of these the probability that this was a chance finding was less than 0.01. In 1 case with 79 consecutive odd series of interectopic beats the chance of such an occurrence was 2<sup>-79</sup>. These results indicate that a predictable pattern may exist in the arrhythmias associated with some cases of acute myocardial infarction.