Experimental and clinical studies indicate a basic linkage between T-wave alternans (TWA) and susceptibility to malignant arrhythmias. In a variety of clinical populations with elevated risk of ventricular tachyarrhythmias, Fast Fourier Transform (FFT)-based assessment of TWA during fixed-rate atrial pacing or bicycle ergometry has shown predictive ability for arrhythmic events. However, after more than a decade since the introduction of TWA testing in human subjects, few studies have explored its utility in ambulatory ECG (AECG) recordings. This gap probably relates to major technical obstacles associated with monitoring of ambulatory subjects, including motion artifact and the requirement of data stationarity, which mandates fixing heart rate. To circumvent these difficulties, we devised a time-domain method, "Modified Moving Average Beat Analysis" (MMA) to determine TWA level accurately in freely moving subjects. Recently, MMA analysis was employed to analyze ambulatory ECG (AECG) records of post-myocardial infarction patients who were were at low risk of arrhythmic death. An increased risk of arrhythmic death was predicted by TWA level above the 75th percentile of controls (p<.05). Thus, the predictive power of TWA obtained with MMA analysis from AECG records obtained appears promising.