Polymorphism of the dopamine transporter genotype (DAT1) confers a small but significant susceptibility to attention deficit hyperactivity disorder (ADHD). We examined whether the volume of the head of caudate, a striatal structure with high DAT expression that is important for inhibitory function, differs by DAT1 in children diagnosed with the disorder relative to age and IQ matched controls. Volume of the head of caudate was delineated in the right and left hemisphere and compared between 7- and 13-year-old children with and without ADHD (combined type) who were carriers of two (10/10) or one (9/10) copy of the 10-repeat DAT1 allele. Caudate volumes were overall smaller in 10/10 than 9/10 children, particularly in the left than right hemisphere. While DAT1 effects did not vary by ADHD diagnosis, overall caudate volumes were smaller in ADHD relative to control children. Altered caudate development associated with 10-repeat homozygosity of DAT1 may contribute susceptibility to ADHD. © 2010 Wiley-Liss, Inc.