The objective of the present study was to investigate the microstructure and the macrostructure of the corpus callosum (CC) in adults with Attention-Deficit/Hyperactivity Disorder (ADHD) by means of magnetic resonance imaging (MRI). Twenty-nine participants with ADHD and 37 controls were included from the Norwegian ADHD project in Bergen. We measured the fractional anisotropy (FA) values, as well as the size of different subdivisions of the CC, using diffusion tensor imaging (DTI) and anatomical MRI. The isthmus/splenium part of the CC in the ADHD group showed reduced FA values compared to the control group, whereas the size of the CC did not differ across groups. Our findings thus demonstrate a divergence between microstructural and macrostructural measures in the CC of adults with ADHD. This contrasts with findings in children demonstrating callosal abnormalities in both microstructure and macrostructure. Our results may indicate that adults with ADHD in part have succeeded in passing by an earlier developmental delay of the CC, resulting in a normalization of callosal macrostructure into adulthood. However, microstructural differences are still present in adults, which may point to an abnormal lateralization in adults with ADHD, or could be a sign of a persisting impairment.