Background: ADHD is a common, heritable disorder of childhood. Sex chromosome abnormalities are relatively rare conditions that are sometimes associated with behavioral disorders. Method: The authors present a male child with ADHD and a major de-novo Y chromosome abnormality consisting of deletion of the long arm and duplication of the short arm. It is possible that the Y chromosomal abnormality is causing the ADHD syndrome in this boy. Conclusion: Considering this case and considering that (a) ADHD is more common in boys than in girls, (b) the transmission of some genes associated with ADHD may occur preferentially from fathers rather than from mothers, and (c) ADHD is more common in children with XYY syndrome and Turner's syndrome than in other children, the authors propose that the sex chromosomes may contain risk genes for ADHD.