Diffusion tensor imaging was used to examine developmental changes in the organization of white matter in a large sample of male participants with autism and controls between the ages of 10 and 35 years. Participants with autism had lower fractional anisotropy in areas within and near the corpus callosum and in the right retrolenticular portion of the internal capsule. Only one area, in the posterior limb of the right internal capsule, showed an interaction between age and group. The findings suggest that reductions in the structural integrity of white matter in autism persist into adulthood. These reductions may underlie the behavioral pattern observed in autism, as well as findings of reduced functional connectivity in functional magnetic resonance imaging signal between activating cortical areas.