BACKGROUND: Cardiac rehabilitation with exercise training alters sympathovagal control of heart rate variability (HRV) toward parasympathetic dominance in patients after acute myocardial infarction (MI). However, its effects on HRV in patients after MI with new-onset left ventricular dysfunction are yet unknown. We aimed to investigate the effects of 8 weeks of supervised, high-intensity exercise training on time- and frequency-domain measures of HRV in this selected patient population. METHODS AND RESULTS: Twenty-five men with an acute MI and a low ejection fraction were randomly assigned to enter or not to enter a training program in a regional rehabilitation center. HRV was evaluated before and after 1 and 2 months of training and at 12 months. Maximal exercise testing with respiratory gas exchange was performed at baseline and after training. Resting heart rate decreased (P <. 01) and the percentage of R-R intervals differing >50 ms from the preceding one (pNN50) increased (P <.05) after training. The standard deviation of R-R intervals (SDRR) tended to increase, but frequency-domain indexes remained unchanged. There was a significant decrease in SDRR (P <.05) and high-frequency power (P <.01) at 12 months in untrained patients. Exercise time increased by 38% and maximal oxygen uptake increased by 29% in the training group (P <. 01). CONCLUSIONS: Despite beneficial effects on clinical variables, exercise training did not markedly alter HRV indexes. A significant decrease in SDRR and high-frequency power in the control group suggests an ongoing process of sympathovagal imbalance in favor of sympathetic dominance in untrained patients after MI with new-onset left ventricular dysfunction.