Objectives. Dysregulation in neurotransmitter signalling has been implicated in the aetiology of attention deficit hyperactivity disorder (ADHD). Polymorphisms of the gene encoding dopamine beta hydroxylase ( DBH ) have been reported to be associated with ADHD; however, small sample sizes have led to inconsistency. Methods. We conducted transmission disequilibrium test analysis in 794 nuclear families to examine the relationship between DBH and ADHD. The effects of the ADHD-associated polymorphisms on gene expression were assessed by luciferase reporter assays in a human neuroblastoma cell line, SH-SY5Y. Results. A SNP within the 3′ untranslated region of DBH rs129882 showed a significant association with ADHD (χ 2 = 9.71, p = 0.0018, OR = 1.37). This association remained significant after Bonferroni correction for multiple testing ( p = 0.02). Further, allelic variation in rs129882 significantly impacted luciferase expression. Specifically, the C allele of the ADHD-associated rs129882 SNP produced a 2-fold decrease ( p