Twelve boys with verified diagnosis of severe attention deficit hyperactivity disorder (mean age 9.2± 2.9 years) and 12 control boys were investigated. According to the DSM IV system the patients were positive regarding all aspects. None of the patients received medication at or around the time of the investigation. The boys were asked to tap a button 128 times in a frequency that felt comfortable to them. The investigation was performed with specially designed software. There was no significant difference in the mean or median intervals between tapping, However, range and spread of the tapping intervals were significantly higher in patients with hyperactivity disorder. The results show that these children perform less rhythmically although they were allowed to choose the tapping frequency themselves. There are many possible reasons for these results, ranging from social to genetic. One of the reasons could be due to changes in the genetically codedmolecular clock. In humans there are only three genes identified that code for molecular clocks (CLOCK, PER2, BMAL1). In these genes as well as in other proteins and enzymes involved in the signaling pathway, mutations and polymorphisms have been described that lead to a decreased rhythm in motor timing. Twelve boys with verified diagnosis of severe attention deficit hyperactivity disorder (mean age 9.2 ± 2.9 years) and 12 control boys were investigated. According to the DSM IV system the patients were positive regarding all aspects. time of the investigation. The boys were asked to tap a button 128 times in a frequency that felt comfortable to them. There was no significant difference in the mean or median intervals between tapping, However, range and spread of the tapping intervals were significantly higher in patients with hyperactivity disorder. The results show that these children perform less rhythmically although they were allowed to choose the tapping frequency themselves. There are many possible reasons for these results, ranging from social to genetic. One of the reasons could be due to changes in the genetically coded molecular clock. In humans there are only three genes identified that code for molecular clocks (CLOCK, PER2, BMAL1). In these genes as well as in other proteins and enzymes involved in the signaling pathway, mutations and polymorphisms have been described that lead to a decreased rhythm in motor timing.