['Despite the rational expectation for a survival benefit produced by exercise training among post-myocardial infarction (MI) patients, direct evidence remains elusive. Clinically, changes in autonomic balance toward lower vagal activity have consistently been associated with increased mortality risk; conversely, among both control and post-MI dogs, exercise training improved vagal reflexes and prevented sudden death. Accordingly, we tested the hypothesis that exercise training, if accompanied by a shift toward increased vagal activity of an autonomic marker such as baroreflex sensitivity (BRS), could reduce mortality in post-MI patients.', 'Ninety-five consecutive male patients surviving a first uncomplicated MI were randomly assigned to a 4-week endurance training period or to no training. Age (51+/-8 versus 52+/-8 years), site of MI (anterior 41% versus 43%), left ventricular ejection fraction (52+/-13 versus 51+/-14%), and BRS (7.9+/-5.4 versus 7.9+/-3.4 ms/mm Hg) did not differ between the two groups. After 4 weeks, BRS improved by 26% (P=0.04) in trained patients, whereas it did not change in nontrained patients. During a 10-year follow-up, cardiac mortality among the 16 trained patients who had an exercise-induced increase in BRS >or = 3 ms/mm Hg (responders) was strikingly lower compared with that of the trained patients without such a BRS increase (nonresponders) and that of the nontrained patients (0 of 16 versus 18 of 79 [23%], P=0.04). Cardiac mortality was also lower among responders irrespective of training (4% versus 24%, P=0.04).'] Does exercise-induced increase in baroreflex sensitivity predict improved prognosis after myocardial infarction?