The present study was conceived with the objective to establish a correlation, between Heart Rate Variability(HRV) and Vanderbilt Total Score in children with Attention–deficit/hyperactivity disorder (ADHD) before and after treatment with Methylphenidate. Our study participants depicted a significant reduction inVanderbilt Parent Rating score from the baseline after 12 weeks of methylphenidate treatment indicating a clinical improvement in the subjects. When the changes in HRV were compared with changes in Vanderbilt Total Score Post–therapy with pre–therapy, significant Positive correlation of change in HF (nu) with change in Vanderbilt Total score and a significant negative correlation of changes in LF (nu) and changes in LF/HF ratio with changes in Vanderbilt Total score were observed. This signifies that with the improvement as shown by reduction in Vanderbilt Total Score, the sympathetic domination increases, and parasympathetic domination reduces, and sympathovagal balance shifts towards the sympathetic arm.