The activity of the presynaptic dopamine (DA) transporter (DAT) is critical in mediating the magnitude and duration of dopaminergic signaling in the brain. Multiple genetic studies have found an association between attention deficit hyperactivity disorder (ADHD) and a variable number tandem repeat (VNTR) in the 3'-untranslated region (3'VNTR) of the hDAT gene (SLC6A3), however none of these studies examined the hDAT coding region for polymorphisms. Thus, we sought evidence of polymorphisms in hDAT, focusing on the coding region and splice junctions, utilizing genomic DNA from children diagnosed with ADHD. Two separate ADHD cohorts (N=70 and N=42) were screened and sampled for both status of the 3'VNTR and for common/novel genomic variants. We found evidence of increased DAT variation in African-American subjects as well as in predominantely hyperactive-impulsive probands. Cumulatively, multiple hDAT sequence variants were identified, including five novel variants, as well as one nonsynonymous single nucleotide polymorphism (SNP), converting Ala559 to Val (A559V). A559V was identified in two Caucasian male siblings with ADHD and both subjects were homozygous for the ADHD-associated, 10-repeat 3'VNTR allele. Interestingly, the A559V variant was previously identified in a subject with bipolar disorder [. Molecular Psychiatry 5, 275], a psychiatric disorder that has a significant number of overlapping symptoms with ADHD.