BACKGROUND AND AIMS: We investigated cardiac autonomic function in overweight and obese school-age children. METHODS AND RESULTS: Quantitative cross-sectional study conducted with children (n = 110) of both genders. Children were divided by normal weight (NW; n = 54), overweight (OW; n = 24) and obese (OB; n = 32). Systolic (SBP) and diastolic (DBP) blood pressure and electrocardiograms were recorded and analyzed for heart rate and the heart rate variability (HRV) in time (SDRR, RMSSD, PRR50, SD1 and SD2) and frequency domains (HF, LF and LF/HF). The OB group presented higher SBP (p </= 0.01) and DBP (p </= 0.01). For HRV, the OB group had a lower PRR50 (p </= 0.01) and HF (p </= 0.01), associated with higher LF (p </= 0.01). Moderate negative correlations were found between the HF, BMI (r = -0.37; p </= 0.01) and WC (r = -0.38; p </= 0.01). Positive moderate correlation were found between LF, LF/HF and BMI (LF: r = 0.32; p </= 0.01; LF/HF: r = 0.31; p </= 0.01) and WC (LF: r = 0.34; p </= 0.01; LF/HF: = 0.34; p </= 0.01). Multiple linear regression showed a positive association between body fat and the SDRR (beta: 0.48; CI: 0.2-4.2; p = 0.02). No differences were observed in cardiac electrical activity. CONCLUSION: Children with obesity but not overweight presented higher blood pressure and cardiac autonomic dysfunction, with sympathetic predominance on the heart rate. This fact was positively correlated with BMI and may be considered an important marker for cardiovascular risk in children.