Ventricular arrhythmia is known as a major cause of sudden death in patients with heart disease, especially in patients with myocardial infarct. Programmed electrical stimulation (PES) is used in order to identify patients with high risk of ventricular arrhythmia, but it is invasive. So ventricular late potential is studied, which can be performed safely. Ventricular late potential was measured in the 65 normal subjects, 17 patients with in-hospital period acute myocardial infarction and 29 patients with old myocardial infarction using signal-averaged high resolution EKG, Mac-15. The positive criteria of ventricular late potential was one of the following : The duration of TQRS is more than 120 msec, or the amplitude of RMS is less than 25µV, or the duration of LP 40 is more than 40 msec. The results are as follows : 1) Among 65 normal subjects(male ; 33, female ; 32), total QRS duration(TQRS)was 103.9±8.3msec(mean S.D), terminal 40msec root mean square amplitude(RMS) 47.8±24.3uV and terminal 40msec mean amplitude was 32.5±15.4uV. Variables of ventricular late potential showed no significant difference by age. 10 subjects showed positive ventricular late potential. 2) Among 17 patients with in-hospital period myocardial infarction, there was no significant difference in variables of ventricular late potential between patients with ventricular arrhythmia(3 subject) and patients without ventricular arrhythmia(14 subjects). 3) Among 29 subjects with old myocardial infarction, TQRS showed significant differrence between patients with ventricular arrhythmia(3 subjects) and patients without ventricular arrhythmias(26 subjects). All of the patients with ventricular arrhythmia(100%) and 6 subjects(24%) of the patients without ventricular arrhythmia showed positive ventricular late potential, and the difference was significant statistically between groups(p value<0.05). This showed that ventricular late potential is helpful in predicting the risk of ventricular arrhythmia among patients with old myocardial infarction.