In the last decades, an increase of new paediatric problems requiring medical care like eating disorders, behavioural and attention problems has been observed. Based on the hypothesis that mental illness is accompanied by autonomic dysfunction, we compared measurements of heart rate variability (HRV) in Holter ECG's from children with attention deficit/hyperactivity disorder (ADHD) with those of healthy children and a historical control from 1997. Method: We analysed the HRV parameters SDNN, rMSSD and pNN50 from 24 hours Holter ECG from children (mean age 10.8 years) with ADHD before or during medical therapy with methylphenidate (MPH). These values were compared with aged matched healthy children. Results: Compared to healthy controls ADHD children with and without MPH treatment showed significantly higher mean heart rates, lower pNN50 and lower RMSSD. pNN50 and RMSSD values of healthy children are lower compared to the historical control group from 1997. Conclusion: Compared to healthy children the peak of parasympathic activity measured by the HRV values pNN50 and RMSSD in early adolescence was reduced in children with ADHD. Compared to a historical control these values are also reduced in healthy children. A reduced vagal activity will - within the meaning of W. Porges polyvagal theory - have consequences on the social and emotional development of children. Moreover this observation may have consequences for an enhanced cardiovascular risk.