Attention deficit hyperactivity disorder (ADHD) is a highly heritable disorder. Although the causes of ADHD are unknown, dopaminergic, serotonergic and nor-adrenergic pathways have been strongly implicated. Monoamine Oxidase A (MAOA) is involved in the degradation of all three of these neurotransmitters and therefore has been suggested as a strong candidate gene for ADHD. Animal and human studies have implicated MAOA and 5-HT in impulsive and aggressive behavior. We therefore additionally postulated that MAOA might be associated with a subtype of ADHD where aggressive and impulsive features are especially prominent. We have tested this hypothesis by genotyping two polymorphisms (the 30-bp VNTR in the promoter and the Fnu4HI 941TG) in MAOA that are associated with altered MAOA function. Our sample consisted of 171 British Caucasian children 6-16 years of age fulfilling DSM-III R, DSM-IV or ICD-10 criteria for ADHD/Hyperkinetic Disorder. Using case control analysis and then the TDT, no association was found between these two MAOA polymorphisms and ADHD. Case control analysis of the VNTR showed an association with a subgroup of children with comorbid conduct problems (OR = 2.0, 95% CI = 1.09, 3.5), and TDT analysis indicated a statistical trend toward association. Our findings highlight the importance of phenotype definition and the need for the MAOA VNTR to be further examined.