Dopamine is believed to play a major role in the manifestation of attention deficit hyperactivity disorder (ADHD), which affects 3-6% of school-age children and shows evidence of familiarity. The dopamine D4 receptor, which is preferentially distributed in cortical and limbic regions of the brain, is currently of major interest because of the high degree of functionally relevant variability in its gene (DRD4), and the association of this gene with Novelty Seeking behavior. We examined the variability in the length of a region of DRD4 that contains a 48-bp repeat sequence in children with ADHD and controls matched for ethnicity. ADHD children differed from controls in that the 7-fold repeat form of DRD4 occurred significantly more frequently than in the control sample. This form of the receptor has previously been shown to mediate a blunted intracellular response to dopamine. Although ADHD is likely to be multifactorial in its etiology and its heritability is likely to be polygenetic, the present findings suggest that polymorphic variation in the gene encoding the D4 dopamine receptor may be a contributing factor in the expression of symptoms associated with ADHD.