BACKGROUND: Sudden cardiac death (SCD) risk is elevated following acute myocardial infarction (MI). The time course of SCD susceptibility post-MI requires further investigation. METHODS: In this observational cohort study, we employed state-of-the-art noninvasive ECG techniques to track the daily time course of cardiac electrical instability and autonomic function following ST-segment elevation myocardial infarction (STEMI) and non-STEMI (NSTEMI). Preventice BodyGuardian MINI-EL Holters continuously recorded ECGs for 7 days at hospital discharge and at 40 days for STEMI (N = 5) or at 90 days for NSTEMI patients (N = 5). Cardiac electrical instability was assessed by T-wave alternans (TWA) and T-wave heterogeneity (TWH); autonomic tone was determined by rMSSD-heart rate variability (HRV). RESULTS: TWA was severely elevated (>/=60 muV) in STEMI patients (80 +/- 10.3 muV) at discharge and throughout the first recording period but declined by 50% to 40 +/- 2.3 muV (p = .03) by Day 40 and remained in the normal range (<47 muV). TWH, a related phenomenon analyzed from 12-lead ECGs, was reduced by 63% in the five STEMI patients from discharge to normal (<80 muV) at follow-up (105 +/- 27.3 to 39 +/- 3.3 muV, p < .04) but increased by 65% in a STEMI case (89 to 147 muV), who received a wearable defibrillator vest and later implantable cardioverter defibrillator. In NSTEMI patients, TWA was borderline abnormal (47 +/- 3.3 muV) at discharge and declined by 19% to normal (38 +/- 1.2 muV) by Day 90 (p = .05). An overall reciprocal increase in rMSSD-HRV suggested recovery of vagal tone. CONCLUSIONS: This study provides proof-of-principle for tracking post-MI SCD risk in individual patients with implications for personalized therapy.