BACKGROUND Positive effects of physical training in adults with acquired heart disease have been reported. The role of exercise training in adults with congenital heart disease, however, is less well defined. OBJECTIVE To assess the level of physical activity in adult patients with repaired tetralogy of Fallot (TOF), as well as the safety and effect of exercise training in these patients. METHODS A questionnaire of physical activity was filled upon entering the study by the participants. Eighteen adult patients with repaired TOF were then randomly assigned to participate in a three-month structured exercise program (exercise group, nine patients) or to continue their usual exercise routine (control group, nine patients). Each patient in the exercise group received an individualized exercise program to be performed three times a week, whereas the control group was told to continue living their life as usual. Cardiopulmonary testing was performed in all patients at baseline (before randomization) and at the end of the study. RESULTS No death or incidents occurred during the study period. All patients but one were physically inactive at the onset of the study. There was a significant increase in peak oxygen consumption in the exercise group by the end of the study (22.1 mL/kg/min [6 metabolic equivalents (METS)] versus 24.3 mL/kg/min [7 METS], P=0.049), whereas it remained unchanged in the control group (21.8 mL/kg/min [6 METS] versus 22.1 mL/kg/min [6 METS], P=0.825). CONCLUSIONS In clinically stable adult patients with repaired TOF, a moderate level of exercise training improves aerobic capacity. Regular exercise should be encouraged in these sedentary patients.