Attention deficit hyperactivity disorder (ADHD) is a prevalent psychiatric condition in children and follow up studies have indicated that 22-33% of patients continue to suffer from ADHD during late adolescence and adulthood. Convincing evidence supports the contribution of genetic factors in the etiology of ADHD, and the interaction of the psychostimulants with the dopamine system suggests that dopamine is involved in the pathophysiology. The 7-repeat allele of the 48 base pair repeat of the dopamine D4 receptor gene (DRD4) has been reported, with several replications, to be associated with ADHD in children. We tested for the presence of association between the DRD4 48 base repeat and adult ADHD in two independent samples: one comprised of cases and ethnically matched controls, and the second made up of nuclear families. Each case was assessed using a battery of adult ADHD assessment instruments. The analysis of the 66 cases and 66 controls showed a significantly higher presence of the 7-repeat in the adult ADHD patients vs. controls (chi(2) = 5.65; df = 1; P = 0.01). In the analysis of transmission of DRD4 alleles in 44 nuclear families with the transmission disequilibrium test, a trend was observed toward a increased transmission of the 7-repeat from the heterozygous parents to the affected offspring (chi(2) = 2.00; df = 1; P = 0.15). When the two samples were combined, the overall significance was stronger (N = 110; z = 2.68; P = 0.003). The results of our study suggest a role of the 7-repeat allele in adult subjects suffering from ADHD. This finding is an important continuation of the group of studies that together strongly suggest the involvement of DRD4 in ADHD.