Attention deficit hyperactivity disorder (ADHD) is a developmental syndrome expressed along three domains: inattention, hyperactive-impulsive, and combined type. Several investigations have recently examined the role of the dopamine DRD4 exon III repeat polymorphism in ADHD. The long 7 repeat allele of this receptor was shown in three family-based studies, but not in one case control design, to be a risk factor for this disorder. We now report an additional family-based study of DRD4 exon III repeat region and ADHD. However, in the current study we fail to observe preferential transmission of the DRD4 exon III long 7 repeat allele, chi(2) = 0. 142, P < 0.1, df = 1. Nor was any preferential transmission observed when genotypes were compared, chi(2) = 0.180, P > 0.1, df = 1. Possible reasons are discussed, especially lack of sufficient power in analying more refined phenotypes, why the current results in contrast to previous findings fail to support a role for the long form of the DRD4 receptor as a putative risk factor for ADHD.