Objective: The aim of the present study was to evaluate heart rate variability (HRV) in children newly diagnosed as attention-deficit/hyperactivity disorder (ADHD). Methods: A total of 51 children with new ADHD diagnosis who were not received any treatment formed the study group and 51 age and sex-matched healthy children were enrolled as the control group. 24-hour heart rate (HR) recordings were performed with rhythm Hotter monitoring and HRV parameters indicating autonomous nervous system (ANS) functions were evaluated. Children in ADHD group were further divided into two groups as 'severe' and 'mild' ADHD according to Clinical Global lmpressions-ADHD-Severity scale. Results: HRV parameters were comparable between ADHD and control groups. Whereas, percentage of consecutive NN intervals over 50 msn (pNN50) and minimum Spectral Power per hour (minSPH) values were lower; while, maximum 1-hour heart rate Hotter (maxHRH) and mean heart rate (HR) values were higher in severe ADHD group than those of control group. Conclusion: These results support that as the severity of ADHD increases, ANS dysfunction becomes more overt. Further large scale, multi-centered, randomized-controlled clinical trials are needed to clarify possible role of ANS dysfunction in ADHD etiopathogenesis.