Background: Previous diffusion tensor imaging (DTI) studies in patients with obsessive?compulsive disorder (OCD) have reported inconsistent findings, and it is not known whether observed findings are related to abnormalities in axonal structure or myelination. Methods: In this DTI study, we investigated fractional anisotropy, as well as axial and radial diffusivity, in 21 patients with OCD and 29 healthy controls. Results: We found decreased fractional anisotropy in the body of the corpus callosum in the OCD group, which was underpinned by increased radial diffusivity. Limitations: The cross-sectional design was the main limitation. Conclusion: Our findings of increased radial diffusivity provide preliminary evidence for abnormal myelination in patients with OCD.