OBJECTIVE: To investigate the effects of an exercise-training program on heart-rate-variability (HRV) indices in individuals with Down syndrome (DS). DESIGN: Controlled clinical trial. PARTICIPANTS: 10 people with DS, age 24.2 +/- 5.1 y (group A), and 10 age-matched healthy sedentary individuals (group B). METHOD: At baseline all subjects underwent a clinical examination and an ambulatory 24-h Holter monitoring for the evaluation of cardiac autonomic-nervous-system (ANS) activity by time- and frequency-domain analysis. INTERVENTION: After initial evaluation, group A followed a 6-mo exercise-training program and thereafter underwent the same HRV analysis. RESULTS: At the beginning of the study, group A showed a higher LF:HF ratio than group B, indicating impaired sympathovagal balance, likely because of lesser vagal modulation. Moreover, both time- and frequency-domain indices in group A were significantly lower than in group B. At the end of the study, exercise training was found to improve the sympathovagal balance, mainly by increasing vagal activity, in group A. CONCLUSION: The results indicate that individuals with DS have ANS dysfunction that can be improved by exercise\'s increasing the parasympathetic modulation.