One of the most consistently reported brain regions of structural and functional difference in attention deficit hyperactivity disorder (ADHD) is the basal ganglia, particularly the caudate nucleus. Examining the structural organization of the basal ganglia in ADHD is important because it is the center of wider fronto-striatal networks, reported to be dysfunctional in ADHD. Fifteen right-handed 8- to 18-year-old males with ADHD-combined type and 15 right-handed, age- and performance IQ-matched healthy males underwent diffusion tensor imaging. Caudate, putamen and thalamus were manually identified as regions of interest (ROIs) and tested for differences in fractional anisotropy and mean diffusivity. Measures of fractional anisotropy (FA) showed the expected increase with age within the whole-brain volume and within putamen and thalamus ROIs for both ADHD and control groups. In the caudate nucleus, however, developmental changes in FA with age were significantly different between ADHD and control groups. This study shows that the developmental trajectory of micro-structural organization within the caudate nucleus is different in children with ADHD compared with controls over ages 8-18 years. We suggest that the difference in developmental trajectories arises predominantly during mid-late adolescence and may reflect a developmental delay that begins to normalise over this critical late adolescent age.