Investigations into the genetic basis of attention-deficit/hyperactivity disorder (ADHD) continue to yield compelling results as candidate gene studies reveal more information about this elusive disorder. Family, twin, and adoption studies further the notion that ADHD is a highly heritable disorder with direct genetic and environmental influence. The year 2005 saw many ADHD candidate gene studies, with most focused on the catecholaminergic candidates. Although many genes were studied in 2005, a large portion of findings has been supportive of the roles of dopaminergic genes' relationship to clinical phenotypes and drug response. These studies often require replication. Clinical implications continue to be speculative, as larger sample sizes are needed to validate findings to the general population. Further understanding of endophenotypes and the impact of comorbidities also is necessary for proper clinical intervention. Forthwith, we provide a summary of ADHD genetic studies published in 2005.