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White matter abnormalities in bipolar disorder and schizophrenia detected using diffusion tensor magnetic resonance imaging Bipolar disorder (BD) is strongly associated with white matter abnormalities. Postmortem studies have revealed a reduction in glial cells in the subgenual prefrontal cortex (1) and there is evidence from T 2 -weighted magnetic resonance imaging (MRI) that patients with BD have an increase in white matter hyperintensities compared to healthy controls (2, 3). T 1 -weighted MRI has also revealed white matter reductions using both region of interest (ROI) (4, 5) and voxel-based morphometry (VBM) approaches (6).Conventional T 1 -weighted MRI is, however, limited in its ability to identify abnormalities of white matter, since the signal is largely dependent upon water content and is potentially confounded by non-neuronal tissue components. Diffusion tensor imaging (DTI) is a more recent technique that measures the diffusivity of water molecules within tissues in vivo. Yet there have been few DTI studies in BD and all of these have had relatively small sample sizes. Their findings have included Objectives: Strong qualitative and quantitative evidence exists of white matter abnormalities in both schizophrenia and bipolar disorder (BD). Diffusion tensor imaging (DTI) studies suggest altered connectivity in both disorders. We aim to address the diagnostic specificity of white matter abnormalities in these disorders.Methods: DTI was used to assess white matter integrity in clinically stable patients with familial BD (n = 42) and familial schizophrenia (n = 28), and in controls (n = 38). Differences in fractional anisotropy (FA) were measured using voxel-based morphometry and automated region of interest analysis.Results: Reduced FA was found in the anterior limb of the internal capsule (ALIC), anterior thalamic radiation (ATR), and in the region of the uncinate fasciculus in patients with BD and those with schizophrenia compared with controls. A direct comparison between patient groups found no significant differences in these regions. None of the findings were associated with psychotropic medication.Conclusions: Reduced integrity of the ALIC, uncinate fasciculus, and ATR regions is common to both schizophrenia and BD. These results imply an overlap in white matter pathology, possibly relating to risk factors common to both disorders.