We estimated the effects of 40 bp variable number of tandem repeat (VNTR) polymorphism located in the 3′ untranslated region (3′ UTR) (9-repeat vs 10-repeat allele) of the SLC6A3 gene, ADHD diagnosis, age, and their interactions as predictors of cognitive performance. Here, SLC6A3 is linked to attention deficit-hyperactivity disorder.