en Cognitive Neuroscience Cognitive Neuroscience Original Original <strong>Introduction:</strong> Neuroimaging evidence has shown the relationship of intelligence with several structural brain properties in normal individuals. However, this relationship with attention deficit hyperactivity disorder (ADHD) needs to be investigated.<br> <strong>Methods:</strong> We estimated grey matter (GM) density of the brain using magnetic resonance imaging (MRI) scan on 56 ADHD individuals, including 30 combined individuals (Mean&plusmn;SD age: 10.44&plusmn;2.41, intelligence quotient: [IQ]=112.13&plusmn;13.15, male, 24 right hands) and 26 inattentive individuals (mean age =11.39&plusmn;2.1, IQ=107.44&plusmn;13.98, male, 28 right hands) as well as 30 IQ matched healthy control group (mean age=11.08&plusmn;2.15, IQ=115&plusmn;13.56, male, 23 right hands).<br> <strong>Results: </strong>In this study, two statistical approaches were used. In the first approach, region-based as well as the whole association patterns between full-scale IQ and GM were computed and compared between groups. The second approach was to examine the differential pattern of GM density without considering IQ in three groups.<br> <strong>Conclusion:</strong> Results showed significant differences between the ADHD group and the control. This finding could indicate that intelligence is not purely based on the density of GM in certain brain regions; it is a dynamic phenomenon and drastically changes neurodevelopmental disorders. Attention deficit hyperactivity disorder (ADHD), Magnetic Resonance Imaging, Intelligence, Grey matter density 551 572 http://bcn.iums.ac.ir/browse.php?a_code=A-10-2244-1&slc_lang=en&sid=1 Farnaz Faridi farnaz1358@gmail.com 13700319475328460041937 13700319475328460041937 No Institute for Cognitive and Brain Sciences, Shahid Beheshti University, Tehran, Iran. Ashkan Alvand اشکان الوند 13700319475328460041938 13700319475328460041938 No Institute for Cognitive and Brain Sciences, Shahid Beheshti University, Tehran, Iran. Reza Khosrowabadi رضا خسروآبادی r_khosroabadi@sbu.ac.ir 13700319475328460041939 13700319475328460041939 Yes Institute for Cognitive and Brain Sciences, Shahid Beheshti University, Tehran, Iran.