Attention deficit hyperactivity disorder (ADHD), a common behavior disorder of childhood, is a highly heterogeneous disease frequently accompanied by various mental disorders, including disruptive behavior disorder (DBD). Studies show that children suffering from ADHD with DBD are at higher risk of antisocial personality, substance abuse, and social adaptations disorder at their adulthood. The dopamine beta hydroxylase (DbetaH) is the key enzyme to ADHD since it catalyzes the conversion of dopamine to norepinephrine, and dysfunction there of is believed to be one of the causes of the disorder. To explore the association between DBH gene and ADHD complicated with or without DBD, the authors analyzed the transmission of a novel polymorphism DBH -1021C-->T, which is found associated with plasma DbetaH activity, in ADHD nuclear families using transmission disequilibrium test (TDT).Consensus diagnoses were based on the DSM-IV. The samples included those from 292 Chinese Han nuclear families with ADHD probands. Genotypes of DBH -1021C-->T polymorphism were determined by PCR amplification, endonuclease digesting and electrophoresis. The transmission of DBH -1021C-->T polymorphism in ADHD nuclear families with or without DBD was analyzed by TDT.The results showed that there was transmission disequilibrium between DBH-1021C-->T polymorphism and ADHD with or without DBD. In ADHD comorbid with DBD, T allele was preferentially transmitted (P T polymorphism in three subtypes of ADHD, which may suggest that there is a more intense relationship between DBD and ADHD-C subtype.