PURPOSE: Cardiac autonomic dysfunction (CADysf) in children is often associated to obesity and may be attenuated by physical activity. In this study, we investigated the effects of resistance training (RT) upon CADysf assessed by heart rate variability (HRV) in obese adolescents. METHOD: Volunteers were assigned into groups according to standard deviation scores for body mass index (z-BMI) and percentile for age and sex: obese (OB; z-BMI from 2 to 3 and >/= 95th percentile, n = 24) and normal weight controls (CG; z-BMI from -2-1 and < 85th percentile, n = 20). OB performed isolated RT during 12 weeks [3 sets of 6-10reps with 70-85% 10RM]. Waist circumference, systolic/diastolic blood pressures (SBP/DBP), lipids, and HRV were assessed at baseline. Only OB underwent postintervention assessments. RESULTS: At baseline, SBP (122.4 +/- 9.1 vs. 109.7 +/- 11.5 mmHg, p < .001) and DBP (76.1 +/- 7.1 vs. 65.3 +/- 5.9 mmHg, p < .001) were higher, while parasympathetic HRV indexes were lower (p < .05) in OB compared with CG. After RT, waist circumference (3%, p < .001) and SBP (10%, p < .001) reduced in OB. Parasympathetic indexes of HRV increased in OB (SDNN: 25%, p = .03; rMSSD: 48%, p = .0006; pNN50: 67%, p = .001; total power: 54%, p = .01; HF: 101%, p = .001) and baseline differences between groups for sympathetic and parasympathetic activities were no longer observed after RT. CONCLUSION: RT attenuated CAdyfs and BP in obese adolescents, by increasing parasympathetic activity and decreasing sympatho-vagal balance.